I have long ago lost count of the number of fireplaces I've seen whose owners can't remember ever cleaning the glass.  It's something simple to do once you know how, that makes a huge difference in the appearance of your architectural focal point.  I'm going to speak generally here, this will not apply to all fireplaces.  But to make it a bit more specific I'm going to talk about fireplaces that are more than or less than 15 years old.  I'm not sure exactly, but that seems to be roughly the cutoff when manufacturers moved to spring loaded systems designed to release the pressure in a hard start.  So let's start with the older fireplaces.

Oh, and before we begin, please make sure to turn your fireplace off well before you start.  Some fireplaces can stay hot for an hour or more after you turn them off, we don't want any sizzles.  And grab a towel or blanket to set the glass on, there's also no reason to scratch anything up. 

If your fireplace was manufactured before about 2007, it will likely have louvered grills on the top and bottom, and it probably won't have a mesh screen on it.  First thing to do is remove those louvers.  The top one likely will just lift up and out, and the bottom one should flip down from the top, you may have to push it in first like and old drawer.  Once you have those off, you can make sure the pilot is turned all the way off if it's a standing pilot fireplace.  Most of these fireplaces will have one of two systems.  They may have wing-nuts around the glass that are quite obvious, and need to be removed, or they may have a clasp like you'd find on an old steamer trunk or briefcase.  Those are usually above the glass, but sometimes below.  Once you find those and open them up, the glass should swing forward from the top, and then lift out of the lip that's holding it at the bottom.  That's the hard part done.  Now generally these were made of tempered glass, so they're basically the same as a kitchen window.  If you clean it regularly, most any cleaner will do.  If you haven't cleaned it for 10 years and the fog is really baked on, you may need a cream type fireplace glass cleaner.  Spread that on, let it sit for 5-10 minutes, then wipe it away.  If it's still not doing the job, put more cleaner on it, and take a brillo pad to it, or the scrubby side of a sponge.  All done, make sure it's good and dry, set it back in place, and you're done.  Once you know how to do it it shouldn't take more than 15 minutes to do.

If your fireplace was manufactured after about 2007, it may still have the louvers, but it may also have a face plate that holds a metal mesh screen.  If it has the screen, you'll grip it on the sides, fairly low, then lift it and pull it out.  Most will just have a hook system on the top that goes over a shoulder bolt and holds the screen on with gravity.  Some you'll have to pull out the bottom first and then lift it off the hook.  Now, these fireplaces will have a springy clasp device, so the glass can push away and open a gap for pressure to release if the ignition doesn't go smoothly.  Most of those clasps are meant to be manipulated with your fingers, but there are some that need a tool.  Always keep your manual, and any pieces that come with it, for such situations.  You'll pull the clasp toward you, then lift it off the tab that it's holding onto, then release.  There are usually just a couple on the top, but there are some models that have some on the bottom too.  Once it's released, let the top drop forward, then grab it on the sides and lift it out.  With newer fireplaces you always want to use an ammonia-free cleaner.  Many will have a ceramic glass, which radiates heat better than the old tempered glass.  Ammonia is a pretty potent chemical, and ceramic glass is vulnerable to it.  It can leave scratches, or eventually cloud over the surface permanently.  So it's best to get a ceramic glass cleaner from your local fireplace dealer, or online.  To set the glass back in, first slot it in on the bottom, there's going to be some sort of lip that holds it in place, then push the top into place.  Now you can grab those clasps again, pull them forward, and drop them down onto the tab.  Replace the screen, and that's that. 

It's best to clean the fireplace glass at least once a year, but more if you're picky.  It's really the only maintenance you should really be doing on your gas fireplace by yourself, so conquer your fears and get after it.  If you're stuck, find the model number down near the gas valve, and type it into Google.  That should get you the manual, which always has a section for how to remove the glass.  Good luck, stay shiny, and stay cozy.