I do use this on OS X and agree that it does not look quite 'native'. It might be possible to port Textadept to be fully native. Scite, which uses the same Scintilla engine, is available as a native OS X app.
It might also be possible to tweak the GTK layouts for the OS X version.
I don't think sticking with GTK for the OS X version is a wise idea. It really just needs to go Cocoa or not bother, since right now it's downright unpleasant to use for me. Scrolling through text using a trackpad, for example, feels completely unnatural next to the rest of the OS X UI (horizontal scrolling appears to barely work at all - vertical scrolling isn't too bad but there is something very off about it). I think the decision to go with GTK for the OS X version was just a poor choice and probably not one made by anyone who uses OS X often.
In the meantime, I guess I'll go look at the source and see what kind of changes would need to be made to use Cocoa in place of GTK on OS X.
It might also be possible to tweak the GTK layouts for the OS X version.