One other thing to watch out for...maybe this is rare but it happened in my case. The tensioner adjusting bolt had been overtightened in an attempt to tighten a stretched belt. The overtightening of the bolt actually bent the ears on the tensioner "bracket" rendering it useless to sufficiently tension a new belt. The bracket is made out of pretty soft metal, so be careful. I tried beating mine back into shape with a hammer before giving up and pulling a tensioner bracket from a salvage G that I had access to.

If you're getting the correct belt tension with the gauge, then most likely not a bent bracket in your case, but wanted to post for others.

Belt needs tightening, If its older than 2 years it needs replacement. Check all your pulleys including your Idler pulley the bearings might have worn out.