“Could not find a valid gem in any repository” (rubygame and others)

Check if you have “https://rubygems.org/” as a source to find gems at:

$ gem sources
*** CURRENT SOURCES ***

https://rubygems.org/

If not, you should be able to add it with

$ gem sources --add https://rubygems.org/
https://rubygems.org/ added to sources

Here are docs for the gem source command.