The word GM is more preferable to me than DM, as the term DM infers that the only responsibilities in the game are to run a group of players through a dungeon. Some players and GM/DMs prefer an entire campaign around one mega-dungeon. In a way, this makes things a little easier for everybody, there is one locale you as a GM/DM prepare for... Wandering Monster tables, check! Maps, check! Room descriptions check! For the player, the combat prep is basically close quarters, although there are a few dungeons that throw a curve ball in the mix. For example, fighting on multiple levitating platforms. A DM may also prepare a town or city where the players can offload their loot and engage in a little r&r. When I think of the term DM, all of the above applies. The Dungeon Master, the creator of dungeons, populator of the labyrinth. And hey, if that's your gig, that's great!
But does a GM/DM just run a dungeon every game? This brings us to the next title the GM, or Game Master.
A Game Master infers so much more, you have entire worlds you create, or at least the part where the players adventure in. Players can go anywhere! (And often do, much to the GM's chagrin) There are continents, kingdoms and/or countries, geographical wonders, and yes, ruined civilizations and eventually a dungeon or two (or three). There are the alternate planes of reality and existence you have to worry about. Then there's monster placement, what critters inhabit them there hills? In a country you need to think about the culture and the people/NPCs the players interact with (although technically you could have a dungeon inhabited with nothing but NPCs). And that's not all! Languages, seasons, earthquakes, flooding, bird migrations, (Well maybe not, unless you're running a Holy Grail RPG) coinage, national economies, bartering, social upheavals, etc. etc.
If you're playing a sci-fi RPG it gets more complicated! You have entire galaxies to plan, then solar systems, then planets, then nations, and on down the line. Galactic civilizations! Alien races... and monsters (really just a new alien species of critter). Technological advancements, do they have laser rifles or sling shots? Teleportation devices or gas powered vehicles? Or a 9 legged oxen?? And the list goes on and on.
Ah yes, all of these responsibilities are the in the lap of a GM or Game Master! Not a Dungeon Master!
However, which one sounds cooler!? DM of course, hands down! GM also implies that you're a corporate SOB, taking away a 50k plus salary running a store, getting cussed at by your fellow employees or underlings in General Manager speak. You get stressed out because 2 of your not so reliable employees called in sick on the same day, so who is going to pick up the slack!? Huh? So GM definitely does not sound cool, not cool by a long shot!
DM just sounds so much better. Not to mention it's almost like saying a shortened version of 'DAMN!' Like as in 'damn' that girl is hot! Or 'damn' when you realize your GM (you know which type of GM I'm talking about, or do you??) just caught you spray painting his desk because this week he was being a really obnoxious prick.
So really, while GM is a more accurate term for running a game, it still sounds obnoxious. DM is more on the side 'Dude, I don't just run dungeons damn you!' On the other hand, 'Damn yo! (Or DM yo!) That sounds bitchin!' rolls off the tongue so much smoother.
It's all a matter of semantics really. Damn!