Etiquette in bumping up a game you're in to your GM?
Unfortunately, there are far too many reasons that can draw a GM away from his games. In my case, I had gallbladder surgery 3 months ago that completely shot my metabolism to hell. Not sure I'll ever get it straightened out, but I get too tired to post on a lot of days.
From what I've experienced on various PbP sites, the generally accepted posting rate is every other day. While not every player can post that often, GMs are usually flexible on the matter. And, of course, some players can post every day (or 2-3 times a day). The problem with that is when something keeps a player from meeting that rate, it can disrupt the game more than it should.
The unofficial '2-day rule' means that if you are waiting on a player to post for his character because the scene requires a response or action, it's usually considered reasonable to move the scene on. It's often okay to assume an action or agreement from the other character, but do it by inference as much as possible--don't post actual actions or especially put words in his mouth.
In the case of the GM's lack of posting, this still holds true, but it's a finer line to toe. The best way around that is post another action for your character... flashbacks work well, as do speculations on the scene, the enemy, fellow adventurers, etc. Or have a dialog with another PC. The point, if you don't want the game to die, is to just keep posting. Don't just let it sit there. Not only do the other players slip away then, but the GM may think there's no more interest in his game.
Even with all that, there are some things you can't get around. I know there was one Star Trek game where the GM vanished. Found out through another player in another game that knew the GM personally that the GM had died (he'd been in poor health for awhile). THAT is Real Life. Nothing you can do about it. However, if you've got enough good players and an entertaining game, you might want to encourage the GM to get a co-GM... for those times when something bad happens.
At the very least, he can change the proprietary nature of the game to not be Sole Ownership. That way, if he has to bow out, someone else could still take over as GM.
Remember: keep posting! Don't let it die if you don't want.