Given that a Mac version of Torchlight is available, will GOG be offering the Mac version for download anytime soon. I know it's available on Steam, but I would much rather support GOG, and buy it here.

I can't remember 100% for certain, but I do seem to recall that the Mac version of Torchlight 1 is Steam-only for some reason. So if you want to play it on a Mac I think you might have to buy it there -- GOG may not have been given the rights to distribute the Mac version.
Why things would be done this way I don't know, but I think that's what's happening.

The Humble Bundle version also includes a Mac build, so I guess it will eventually land here as well, probably in a month or two.

Oh, I didn't realize that. I thought it was just the PC + Linux versions that the HB release had.
Well, that's good news for GOG's Mac users then.