Hello [name]! Thanks for joining the GMC. Before making any posts in the Tech Support forum, can we suggest you read the forum rules? These are simple guidelines that we ask you to follow so that you can get the best help possible for your issue.
You can use one computer at a time unless you have a multi user site license, which is something I'm not sure GMS has?? You can however use it on multiple computers on Steam just not more than 1 at a time.