Looking at the Greek you provided, the word "of" is not included in the text ... and the word "in" does appear in the text:
I know, but an "in" is in the part of the verse that says "by faith in Christ". So, if an "in" was intended, it would be there. And while I'm not an expert in Greek, I have observed the same thing in many other places in the Bible, so I think "of" is an
appropriate interpretation. See below
[Rom 15:32 KJV] 32 That[G2443] I may come[G2064] unto[G4314] you[G5209] with[G1722] joy[G5479] by[G1223] the will[G2307]
of God,[G2316] and[G2532] may[G4875] with you[G5213] be refreshed.[G4875]
[3Jo 1:3 KJV] 3 For[G1063] I rejoiced[G5463] greatly,[G3029] when the brethren[G80] came[G2064] and[G2532] testified[G3140]
of the truth[G225] that is in thee,[G4675] even as[G2531] thou[G4771] walkest[G4043] in[G1722] the truth.[G225]
[Heb 11:25 KJV] 25 Choosing[G138] rather[G3123] to suffer affliction[G4778] with the people[G2992]
of God,[G2316] than[G2228] to enjoy the pleasures[G2192][G619]
of sin[G266] for a season;[G4340]
Here is Gal 2:16 again:
[Gal 2:16 KJV] 16 Knowing[G1492] that[G3754] a man[G444] is[G1344] not[G3756] justified[G1344] by[G1537] the works[G2041] of the law,[G3551] but[G3362] by[G1223] the faith[G4102]
of Jesus[G2424] Christ,[G5547] even[G2532] we[G2249] have
believed[G4100] in[G1519] Jesus[G2424] Christ,[G5547] that[G2443] we might be justified[G1344] by[G1537] the faith[G4102] of Christ,[G5547] and[G2532] not[G3756] by[G1537] the works[G2041] of the law:[G3551] for[G1360] by[G1537] the works[G2041] of the law[G3551] shall[G1344] no[G3756][G3956] flesh[G4561] be justified.[G1344]