I asked C. about it. "You'd never use 'to bone up' in British", he said, quite reasonably. His mother disagreed, suggesting too that they don't mean the same thing. She thought that "to gen up" involves gathering general information on something. "Boning up" involves a more detailed study of something, becoming a bit more of a an expert on a particular topic.
So, in honor of International Post-a-Poll Day (singlehandedly declared by easterbunny), here is a poll on the subject.
To gen up?
To bone up?
And do they have different meanings, for those of you familiar with both phrases?