I have two ssd hard drives that has a switch as to have only 1 on at a time on so I can dual boot (windows 7, Windows 10) and they don't recognize each other.
My question is if I installed the game in Windows 10 under D:\game\gog.com\ and when I boot in windows 7 the game isn't installed but the files are sitll there, do I need to install the game again in win7 or just create a shortcut to run the game?

Depends on the game. Try launching it and see if it works.

@Maighstir has it there. It depends on the game. Some games require various directories to be present in certain areas for save, ini files, etc. Some require registry entries. Some games however are totally portable and you can pick up the install folder and put it anywhere - Diablo for instance runs nice off my pen drive, no install anywhere. Whether it runs under Win10 however is another question.

There's also the question of dependencies and runtimes. If those aren't present they need to be installed.

If the game is DOSBOX based, it will probably run fine. If it's a windows game, you have a 30/70, where 30% is your likely 'will run fine'. I'm quite annoyed at how windows, the registry and everything embedding itself into the OS to get things to work.

Ok Thanks

I think GOG DOSBox games are actually a bit tricky, especially the CD based ones, because the shortcuts can be quite detailed, also pointing to the correct dosbox config file etc. For instance, here is one shortcut to IHNMAIMS;

"C:\GOG Games\I Have No Mouth, and I Must Scream\DOSBOX\DOSBox.exe" -conf "..\dosboxnomouth.conf" -conf "..\dosboxnomouth_single.conf" -noconsole -c exit

As long as you get that shortcut right (e.g. copy it as well, and check that it points to the correct game directory), then the rest of the DOSBox game should be fully portable and self-contained.

EDIT: Scratch that, apparently that same shortcut is also included in the game directory as well, so you don't necessarily need the separate Start menu shortcut? Good thinking, GOG! That should indeed make them fully portable.

Your main issue will be dependencies. Most games will install libraries for C++, DirectX and so on. If they're already installed by another game, you're good to go. Otherwise it will error.

Google the error and you should find the install for the missing libraries.