First let me say this is just a simple curiosity. I got myself an old Mac Mini with a Mac OS X 10.5, and was wondering if there might still be some games on GoG available for this OS. Probably not, but I do have a retail version of Diablo 2 that I can play on this OS, so who knows...

There is not a single game that GOG officially supports for Mac OS X 10.5 and their Mac OS X minimum has never been below 10.6.8, not even when they first introduced official Mac support back in October 2012. Having said that, there is no guarantee that the Mac builds won't run on your computer, but if they don't run, you won't be eligible for a refund.

Now, as to what could possibly run, I would make a guess and say all the games that run on DOSBox or [url=http://www.an-ovel.com/cgi-bin/magog.cgi?ver=652&scp=gdspu&dsp=ipgfsorlcmbaxyzXhDFGHTP0512348&ord=&flt=xcs~+Scumm~&opt=&myf=ThuDec172028052015_pJ8rZq9BCcjGd]ScummVM[/url]. But, I might be mistaken as I don't own a Mac, so I'd rather leave further answers to those who do.

No problem with that. My main computer's OS is Windows and I play with that one, but got recently this Mac and wanted to at least toy with it a bit and check what I could still do with it.

if you mac is a powerpc mac you really cannot play any gog games, if your mac is intel you can try some games (try with the free games that are wineskin port and they are really old).

by the way with a new mac you can play a lots of game on mac (with wineskin almost all games on gog, also most of windows exclusive games)