gDisk does pretty much what you think it does, which is basically store files in your gmail account. Which is, for the most part, "cool." It works pretty much how you expect it to work, by creating draft messages and attaching the file. It has the concept of "categories" locally, which correspond to labels on the gmail side. Why they don't just say "labels" is beyond me.
But what doesn't it do? It does not act as a mounted drive like it's Windows and Linux counterparts. It's not a huge deal, and I'm glad something like this finally came out for OS X. But, and you knew it was coming, it's just one more app that has to be launched. Having file system integration saves you that much.
Found via digg (which is the new slashdot, and by "slashdot" I mean "annoying community-driven site").