It's also for Civ IV and Civ III.
I think I'm going to get the Civ V Gold Edition as I haven't updated from the base Civ V yet, so this is the perfect opportunity.
I do have a question though for anyone - I've heard that once you buy Civ V from Steam, other items related to it are supposed to be discounted, like for instance I've read that the Gold edition for some people was automatically discounted on Steam because they already had Civ V. This never happened to me and I wonder why not. I bought the retail Steam version (basically a code to enter) but I don't think that should matter. Still, the Gold edition shows up for me only at full price and I thought it did for everyone until reading around this site recently and finding some people saying it's discounted for them (separate from this daily discount, I mean).
Anyway, it doesn't matter much as now I'll get this fantastic 75% off deal - lucky me as I was thinking of updating this month anyway. But still, I'm curious why the discrepancy between me and others about the automatic discounts that I "should" have gotten.
This deal is also making me antsy to just go ahead and get Civ IV on Steam, because it's extremely cheap with the 75% off what was already a cheap price. I already have it with a disc (haven't played it since Civ V came out and actually un-installed it from the computer) but it'd be a nice reminder having it on Steam and I might load it up now and then. I've never played Civ III but even though with the deal it's so cheap as to almost be free, I don't think I'll bother as I've heard Civ IV really is the standard for past games.