There are already tags that may (or may not be) equivalents. How do we want to handle gm vs. dm and online vs. online-resources for example?
I do believe there is a way to make synonyms for tags. That way if someone searches for [GM] they will get [DM] results as well.
I think [dm] should be blacklisted (I dunno if we can do that in beta) as it's kind of DnD specific, while this site is RPG-oriented, not DnD oriented.
If someone tries to use [dm] it should give them a message informing them to use [gm] instead.
Sunno about the rest...
Also, you can just edit the tags. I have arrogantly taken it upon myself to purge all the [dm] tags (changing them into [gm] tags) -- if this is wrong of me let me know and I'll stop. But it seems like a decent solution for now.
I like synonyms; also frankly DM vs GM serves useful segmentation; you know a "DM" question is going to be about D&D and cover certain concerns that are often different from "GM" questions from the broader game world.