why no metal slug 4,5,6,7 available? and what about metal slug 3d, will it be available for pc and gog someday?

It is not gog that took the game Roms, obtained the licenses and wrapped them up in a emulator. It was (as far as I got it) DotEmu. So, I would not hold my breath in any other Metal Slug game, even if all of them already work in emulators.

I doubt it, those games are more steam only games or console ports only. I would love to get it on pc drm free but a man can dream.

My suggestion is to buy Metal Slug Anthology and Metal Slug 3d for the PS2. The latter may be difficult to find.

They work fine via the PCSX2 emulator, with this two games you pretty much have everything a part of Metal Slug 7, as it was for the Nintendo DS.

To play Metal Slug 3d you probably need a controller. For all others, a good keyboard is enough.

Sounds like they just should have ported those versions...

Perhaps? Perhaps not? Do you have the power to decide?

Why not use Anthology and 3d? You don't like PCSX2?