Why are the 15th and 16th moves considered blunders ?

Hi everyone,
Why the 15th & 16th black moves are considered blunders ?
Why the pawn should take instead

Thanks for your useful comments.

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15. ... gxf5 is immediately threatening e4.

It opens up the 6th rank for the rook to come over to either g6 or h6. The black queen is a threat to join the rook after Qd8.

The knight on f3 will need to move, and it has a choice of sucky squares to occupy.

It's the start of a kingside attack for black that will gain space and start to limit white's options.

@ericmsd said in #2:
> 15. ... gxf5 is immediately threatening e4.
>
> It opens up the 6th rank for the rook to come over to either g6 or h6. The black queen is a threat to join the rook after Qd8.
>
> The knight on f3 will need to move, and it has a choice of sucky squares to occupy.
>
> It's the start of a kingside attack for black that will gain space and start to limit white's options.
what he said. merry chrismas!

Also, not recapturing with a pawn means that White can establish a knight unassailably on e4, which--among other things--hems in your bishop somewhat.