Hi i can;t install a new galaxy 2.0 app in my PC.

I install this and show mi the message:

gog galaxy executable checksum doesn't match. please reinstall the application.

I tring reinstall aplication 2 timies and erros still shows.

I cant play my games because of that. Please do somthing.

Unfortunately I can't help you with the Galaxy installation issue. But even if Galaxy isn't working, you can still play your games.
Just go to the folders where the games were installed and run the launch executables. All games - even if they were installed through Galaxy - can still run offline without Galaxy.

You don't need the galaxy client to play your games. Just download them from your account here.
https://www.gog.com/account

Ignore the blue 'Install For Galaxy' button, and use the normal, offline installers underneath.

Did you try to redownload it?

Try to completely delete Galaxy via Win 10 apps in Settings and then redownload Galaxy and install it again. Also try to temporarily pause your anti-virus software when you install Galaxy.

But as others have said: Galaxy is not needed to play GOG games though it can be very useful.

Try to block your antivirus before install, in case the problem continue contact support and use the ofcline installer of the game you can get in gog.com/account