So Romans 11:29 only applies to "spiritual gifts"? What in the verbiage of the verse compels you to think this.
On your question, what is my understanding of the verse?
I believe that the verse is a true but general statement that applies to individuals who have been blessed by God. God is a giving god who desires the best for His followers. But this verse does not prove a person has whatever gift they claim to have.
This is another true but general statement:
Numbers 23:19
New International Version
God is not human, that he should lie, not a human being, that he should change his mind. Does he speak and then not act? Does he promise and not fulfill?
Can we take this verse and claim that God
never changes His mind.
Of course He has, just ask Abraham and Moses.