That soundtrack search results while searching for games are very annoying. How I search for games only?

You don't. The filtering options leave a lot to be desired.

Yep, unfortunately. It would be easy to implement too. Just imagine having filters like exact release year, DLC, proper genres names (there are still too many missing)...

I know how easy it would be to implement decent filters. The whole website reeks of being built by a graphics designer rather than a real developer. I know, I worked for several years with a marketing firm and it's really obvious when a graphic designer has been put in charge of a web design project rather than someone who knows what they're doing.

Excluding soundtracks from a search on GOG would not equate to searching for games only. In order to accomplish a games-only search, they'd also have to exclude costume DLCs, EXP booster DLCs, visual novels, and probably a whole lot of other things too.