Any type of RTV gasket sealant should work, it's optional so you don't have to be dealing with the gasket from moving out of it's position; if you install the bolts at each corner on the pan you can use them to hold the gasket in place and you don't need to use the sealant. If you do use sealant it will also help you to prevent any further leaks
.
It is a good idea to do it yourself first if you have no inconvenience with it, the shop will do the same and will charge you the labor which, as far as I know, in this case is expensive. If you still have problem then take it to the shop.
Best regards,
Oscar.