One of the definitions of "against", taken from Merriam Webster:

> 4 b: in contact with

> "leaning against the wall"

OP's code is "in contact with" the public APIs provided by GTK. That is, OP uses the GTK library.

He means utilizing GTK. In sweden its common to say "Utveckla mot xxx" Develop against, when talking about using a framework or library. I bet they say the same thing i Other germanic languages.

Works in German too but reads somewhat awkward.

Ich habe die Lösung gegen das GTK-framework entwickelt.

It's ambiguous btw.

Maybe they meant programming against GTK, not being against it?

Probably as in 'linking against'.