korell: I wonder, could you try having the Steam copy installed for the single-player campaign and the GOG copy installed for XMP? They'll be in different install folders, but I've no idea if they'll overwrite any registry or shared files.
Anyway, being as you have the Steam copy, have you played through the campaign yet? If so, you might want to get the GOG copy just for its XMP installation. After all, it's not exactly expensive here, and it's in the sale too. :)
Alternatively, you could try downloading the XMP.
http://www.unreal2.com/download_xmp.html Don't know if it will install without a retail disc, though, but it might be worth a try.
ThreeSon: That's basically been my dilemma. I bought the Steam version forever ago, and I owned the retail SE version from before that, but I've been interested in the GOG version because it's the only "complete" version available in a digital format. Ultimately, even though it's on sale it's not really worth it since I still have the disc version on me, and I also doubt there's very many online players running XMP matches right now (or is there?).
I still ended up buying a little over $50 worth of GOG games today, including the entire D&D lineup, so I'm going to be a little short on time for a while anyway. :)
Thanks for your assistance.
In that case, I'd recommend you download the XMP and see if it works with the Steam version. If it does then you get it without having to spend any more, if it doesn't then I'd recommend finding out if the XMP is really something that you want before you buy it from here. I've not actually played it but I did see some posts in these forums saying that the XMP is much better than the single-player campaign.