I'm not sure if any specific "training" is possible. The first thing you might want to do is try to run the rubygems test suite under MacRuby, just to see if it even runs. I tried it, one test crashed, but it finished and here are the results for me:
882 tests, 2489 assertions, 34 failures, 113 errors, 0 skips
Of course, that was mixed with a whole whack of warnings. However, I believe it the test suite couldn't even finish when I tried last (almost a year ago).
Next you might want to find the MacRuby patches to rubygems and see if you can apply them, or if you even need them, on the current rubygems source. You can find the patches if you search the MacRuby lib/ directory for "XXX MACRUBY". If we're lucky, all the tests will pass at that point and you can try building macruby with a new rubygems to see if it actually works.
If not, we would have to evaluate which things to fix; it might not be worthwhile to fix some edge cases right now. Like Matt said, it' s not a simple job...
PROTIP: running the rubygems test suite under macruby seems to mess with your ~/.gemrc file and then doest fix it afterwards, whereas it would restore the original when running the tests under CRuby; now, when rubygems stops working after you run the test suite you'll know why
Hope that helps,
Mark