Gags can hurt sometimes; if it's really filling (like a ball gag or a totally stuffing cloth gag, two socks stuffed into your mouth and taped shut) and strapped tight, they'll make the jaws ache a bit, and you get a dry, parched feeling in your mouth after a while. On the other hand it gives a lot of drool, esp. if your mouth is spread out but not plainly filled (ball gags and ring gags) because you can't keep it in with your lips.

I'm a big fan of ring gags and that one will give lots of drool, on the other hand that's part of what turns your Dominant on, seeing that you can't keep control of your mouth. I guess you'll have to try out, the longest I've worn a gag continuously is around three hours but as with other stuff, you start with a short time to see what it's like and then maybe you'll want to keep it longer.

Rope, belts and scarves etc doesn't hurt in general IMO, but being bound tight and squirming does make you sweat a lot sometimes (also because you're bouind to get turned on and aware of how surrendered you are... ) Being bound with heavy duct tape can hurt, it doesn't feel comfortable to say the least and I've avoided it though r/l robbers sometimes use it because it's around and very hard to get free from if it's run a few times and the victim can't move and reach a pair of scissors or something.