I noticed today that I can actually grab the gasblock on my flot-top upper and MOVE IT back and forth about 1/16" (approximately). Is this a bad thing and can it affect the functioning of my rifle? Unfortunately, I'm pretty sure my warranty has expired on it so is it something I can repair myself?

The gas block shouldn't move.  Is it a pinned front sight or one of the rail type blocks?  If it's pinned, it would indicate that the pins have enlarged the cuts in the barrel in which case it would need to be drilled for larger diameter pins - if it is a rail type, they are usually held in place with a set screw so try to tighten it.  

Whatever the type, you will be losing a degree of gas from the block but not enough to cause a malfuction, just keep your hand away from the gas block when firing.  

Even if the warranty is out, I would imagine the manufacturer would take care of this.  

I have an aluminum gasblock on a flat top...
It blew off after the first couple pops. Nice!
I drilled the set screw holes to a bigger size, tapped them accordingly, and used longer set screws. They stick out a little, but the gas block does not budge.

Well, thanks for the replies. I found the problem WAS in fact a loose set screw on the bottom of the gasblock. The funny thing is that there are two places for them but only one is drilled out. Should I put some Loctite Blue on the screw and torque it down or should I drill out and tap the other hole?

use the other hole also

Loctite as well.  Not a bad thing!