Not too bad, but certainly a big step up from an alternator.

Removing the manifold and replacing the gasket isn't the hard part--it's getting to it (removing the TBI) where you have to be concerned.

You'll need to keep track of all of those vacuum hoses, fuel lines and electrical connections. In addition to a new intake manifold gasket, you'll need a new gasket for the TB. You will definitely need a torque wrench.

I've pulled the intake manifold on a couple of engines, though not a GM 2.8L V6. If you don't already have a repair manual of some sort, now would be a good time to buy one.