My son wanted to get a gel topper for his bed after feeling it on our bed. So we got this twin size GEL topper and put it on his twin futon. He is super happy with it. It's surely 3 inches in thickness and so gelly. Feels like sleeping on cloud. The measurements are accurate to the twin size. 
It took about 16 hours for it to come to it's maximum size after unpacking it as it was vaccum sealed. 

It's a decent price too.