GW, given its age, runs on pretty much anything nowadays. They (Arenanet) stated a while back that they weren't going to support Windows 95 anymore, so that should give you an indication of what I mean. Of note however, is that GW doesn't run natively on non-windows systems, so you need something that helps with that (again, see the above link).
In any case, a free trial is available here. You can always try to get that running.
(Also, if you're referring to GW2, you're in the wrong place. See here instead.)
Beware that GW1 does not have a mac client. To play it on mac you will need http://www.guildwarsguru.com/forum/g...t10488969.html
or something similar.
The issue is that it may be more taxing on hardware so.... You need to try and see whether it works. Best as it was suggested, start with a demo. If it works you may try the full version.