I have a 1988 S10 Blazer with only 49,000 miles on it. As you can tell I don't drive it much. Recently I took it on a long trip of several hundred miles. When I stopped at one point, there was some gas leaking from around the fill tube. When I removed the gas cap a LOT of pressure was released. And I mean a LOT. It was still doing some serious venting after about 10 seconds. I've since left the gas cap a bit loose and sometimes smell gas when I stop and get out. What would cause such extreme pressure?

 

Your vent line to the evap canister is plugged or kinked, or the valve controlling it is inop. Here's a general diagram not specific to your model



Could also be the canister is not being purged into the intake, Often the problem is mechanical but you should get it checked out. IIRC working on my brother's 89 S10 the system wasnt as complicated as they are now.