The gap theory is a valid argument. The text does not state whether there is or isn't a gap. We simply cannot assume that verse two took place immediately after verse one and that is the case for a great deal of scriptures throughout the bible.
The idea of a gap in scripture is proven in the example of the reading of scriptures by Christ concerning his first and second comings. A gap is found in the middle of a verse but is not apparent in the casual reading of that verse yet a gap is there.
Luke 4:16 And he came to Nazareth, where he had been brought up: and, as his custom was, he went into the synagogue on the sabbath day, and stood up for to read.
Luke 4:17 And there was delivered unto him the book of the prophet Esaias. And when he had opened the book, he found the place where it was written,
Luke 4:18 The Spirit of the Lord is upon me, because he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised,
Luke 4:19
To preach the acceptable year of the Lord.
Luke 4:20 And he closed the book, and he gave it again to the minister, and sat down. And the eyes of all them that were in the synagogue were fastened on him.
He stopped reading the entire verse because there would be a gap between the first part and the last part so a gap between the first 69 years and the 70th is not a new concept.
Isaiah 61:2. To proclaim the acceptable year of the Lord, (((GAP IS HERE))) and the day of vengeance of our God; to comfort all that mourn
If a gap is found right there in the midst of a verse then can anyone say it is impossible for a gap of time to be here:
Genesis 1:1 In the beginning God created the heaven and the earth.
(((GAP OF UNKNOWN TIME LENGTH HERE)))
Genesis 1:2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
http://www.blueletterbible.org/tmp_dir/strongs/1143160133-2504.html
Also, the Hebrew word for "was" without form is hayah and the tense that the verb is Qal which means "to become". Therefore the verse actually says "And the earth BECAME without form, and void;" meaning that God originally created the earth complete and that at some point it BECAME void and without form. Some form of destruction took place which strengthens the "gap theory".
01961 hayah {haw-yaw}
a primitive root [compare 01933]; TWOT - 491; v
AV - was, come to pass, came, has been, were happened, become,
pertained, better for thee; 75
1) to be, become, come to pass, exist, happen, fall out
1a) (Qal)
1a1) -----
1a1a) to happen, fall out, occur, take place, come about,
come to pass
1a1b) to come about, come to pass
1a2) to come into being, become
1a2a) to arise, appear, come
1a2b) to become
1a2b1) to become
1a2b2) to become like
1a2b3) to be instituted, be established
1a3) to be
1a3a) to exist, be in existence
1a3b) to abide, remain, continue (with word of place or time)
1a3c) to stand, lie, be in, be at, be situated (with word
of locality)
1a3d) to accompany, be with 1b)
(Niphal)
1b1) to occur, come to pass, be done, be brought about
1b2) to be done, be finished, be gone
In addition, the "mood" of the verb is "perfect" and it affects the meaning in this way: "The Perfect expresses a completed action." This means that the earth was not created void but indeed BECAME void in an action that is completed by the second verse of Genesis.
So, according to the rules of Hebrew verb tenses, it is not "was void" as if it was made that way but it "became void" as the voidness "came into being".
Genesis 1:2 And the earth [BECAME] without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
Scripture has a second witness of this truth:
Isaiah 45:18 For thus saith the LORD that created the heavens; God himself that formed the earth and made it; he hath established it, he created it not in vain, he formed it to be inhabited: I am the LORD; and there is none else.
God did not create the earth void and without form as Genesis is thought to state. God destroyed the world and then recreated everything within it and that's what we read and see in the Genesis account. This is also why scientific evidence that God left behind affirms that the earth is much older than 6000-7000 years.
Zec 9:9 Rejoice greatly, O daughter of Zion; shout, O daughter of Jerusalem: behold, thy King cometh unto thee: he is just, and having salvation; lowly, and riding upon an ass, and upon a colt the foal of an ass.
Verse 9 shows the first arrival of the Messiah
Zec 9:10 And I will cut off the chariot from Ephraim, and the horse from Jerusalem, and the battle bow shall be cut off: and he shall speak peace unto the heathen: and his dominion shall be from sea even to sea, and from the river even to the ends of the earth.
And verse 10 shows the second advent. As of this century there is a gap of time between those two events that is about 2000 plus years and growing daily until he does in fact return.
Verse 9 was fulfilled here:
Mat 21:4 All this was done, that it might be fulfilled which was spoken by the prophet, saying,
Mat 21:5 Tell ye the daughter of Sion, Behold, thy King cometh unto thee, meek, and sitting upon an ass, and a colt the foal of an ass.
Mat 21:6 And the disciples went, and did as Jesus commanded them,
Mat 21:7 And brought the ass, and the colt, and put on them their clothes, and they set him thereon.
Verse 10 won't be fulfilled until the second coming but the OT scriptures do not explain there is a gap of time.
Isa 9:6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
Isa 9:7 Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
Here again is a gap of time within verse 6. Jesus is born, and then there is an unknown period of time before "the government shall be upon his shoulder" and "the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever" which naturally is completed at the second coming or even the eternity after judgement day.
Mat 4:11 Then the devil leaveth him, and, behold, angels came and ministered unto him.
Mat 4:12 Now when Jesus had heard that John was cast into prison, he departed into Galilee;
How much time passed between the angels ministering unto Christ and when he heard about John being imprisoned?
Mar_1:14 Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God,
According to Mark, John was in prison before Jesus returned to Galilee from being tempted in the wilderness so there is an unspoken gap of time between Matthew 4:11 and 4:12