After installing GOG Galaxy I went to run it. It comes up and says the minimum version supported is Windows XP with SP3.

I have WinXP Pro 64 bit, SP3 was never made for 64 bit XP. For 64 bit the last SP was SP2.

There's the question of why are you running Windows XP?

Also, you do realize that 64 bit XP wasn't really a true thing?

And that nothing runs on it anyway.

There's this thing called money. And you need a bunch of it to be able to stop running XP

Because it's really a copy of Server 2003 which raided XP's lipstick cabinet.

Well, yes. But buying the 64bit version of XP would have cost a bit!

Nothing wrong with running XP.
Everything's wrong with running XP64!

Just by putting aside one single dollar each month since Windows XP was released you could by now have bought a brand spanking new copy of any newer version of Windows. Undiscounted.

Or you could buy a gently used PC and go wild with the upgrades.

sounds like seamonkey
its firefox in mozilla drag

You kind of already answered your own question. You might never get it to work on SP2. You should seriously consider upgrading to a newer Windows. XP doesn't even get all security fixes any more. Win7 might be your best choice atm, high compatibility, stable and rather cheap to get.

I would rather think the opposite, Firefox at least tries to look pretty whereas Seamonkey just doesn't care whatsoever and stumbles out the door in the same sweaty t-shirt it's been wearing the past couple of weeks since it last showered.

Actually, for mainline XP, the support status is a manner of none. All support ended in 2014. Yes, even for the embedded products. Save for extended, but you'd be crazy to keep running it at this point.
It amuses me that the various screenshots are for the glossy old versions, rather than El Capitan and 10.

I haven't noticed. But then, I haven't been to the web site in ages either.