Elaboration of a blunder.

My move 15 (white) was a blunder according to the analysis. But it doesn't explain why. I used a lot of time thinking and deciding between moving the G vs. H pawn. I ended up moving randomly, without being confident in my move. (Because of my time issues)
The only reason I can see, is that I could have forced open up the black defensive pawn structure. Which properly would have been more beneficial to me, than it would have been to black?

If anyone can give a more specific reason, please ping me.

@NaturalBornTraveller Your position dropped from "much much better" to just "better", thus the engine classifies it as "blunder". h5 is decisive, cracking black king's position open. Your h-rook is staring right at him, other pieces will follow Your king in the center is weirdly much safer than his. g5 is too slow, you give him the f5 square for his knight and overall more opportunity to bite back.

But of course it isn't really a "blunder"...

g4 is keeping the knight out of f5, while h4 is cramping up your bishop. Moving to g5 lets the opponent's knight start moving while trapping your bishop in place and locking out your knight, while h5 would have done the opposite; given your bishop more space, kept the black knight out, and either opened up the h file for your rook or won your opponent's g pawn.