I purchased a 12" Titan Tools strap wrench off Amazon.

I was able to use it to remove the handguard.

As you can see (please ignore my messy workbench), they indeed used some sort of adhesive liquid to keep the handguard in place. The barrel nut did twist a bit when I was rotating the handguard off, so the gas tube bent. No big deal. I have it off and will now start building an upper for my M16A1. BTW, the set screws on the gas block came off easily. The gas block did need a bit of motivation, but it came off without much trouble. Thanks everyone for your advice.