It would be wise to replace the intake gasket with a new one than to try to fix the old one. How long it would take to replace the gasket would depend on the model, year and engine type you have.
It should take about an hour to change the intake manifold gasket. This part can be replaced by taking off the intake manifold cover, cleaning the old gasket off and sealing a new gasket in the space and tightening the bolts. When other things are in the way, like the alternator, it will take longer.
Several manufacturers built 3800. Without that information no answer can be given
Depends entirely on the year and fuel system. You did not provide that info.
The starter is located under the intake manifold, and you will have to remove the intake manifold to get to it. Do not forget to buy a intake gasket before you take it off, because they always fall apart when you take the intake manifold off.
take it to a mechanic
The only way to get to the intake manifold gasket is to take the head off. I'm working on it right now and so far that is all i can say
depends on the mechanic, should only take like an hour at most.
About 2 hours.
if its a v6 and you take your time. it takes about 6-8 hours, depending on how fast you work !
The year, engine size and if it is the intake manifold or an exhaust manifold gaskets would help.
You shouldn't need to take off the intake manifold at all so, to answer your question, no. However, if it the exhaust manifold you mean, the answer is still no. It should be a multi-layered metal gasket and the only reason to replace any gasket is because it is damaged.
Not sure what an XTL is, but usually when spark plugs are under an intake you should remove the intake to replace them. Don't forget to buy a new intake manifold gasket BEFORE you take the car apart.