"Do I have to learn all of ruby or what?"

Just use what works for you. If you think a concept such as
lambda is too difficult, dont bother using it.
You can survive easily on ruby without it, despite what
others may say about this ;)

For ruby+games I would recommend the SDL bindings of ruby, but
actually the name is "rubygame" on 
http://sourceforge.net/projects/rubygame/
Gosu exists too but i have like 0 experience about this.
(Rubygame is nice btw but I think it could need a little nicer API call, 
hopefully the 3.x branch on it will tackle this)

In my opinion though, game development in general is very time taking :(
I felt left alone kinda, cant people put up the game-logic in .rb files
for all the common games... building a GUI around this is a lot LESS
work than thinking about algorithm, win conditions lose conditions
allowed actions, game logic stuff etc... etc.. :)
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