After all these people tell you it's a bad idea, the FIRST ON YOUR AGENDA is to do it... Go you
Here's an idea, instead of eating a 500 calorie gainer where most of the calories are from unhealthy starch and bad oil, go buy a flat of coconut milk in small cartons, several jars of different nut butters, a few bottles of fish oil, and some protein blends (not pure whey, get blends). If you match the calories with these ingredients, you will gain weight just as well (if not better), have more energy, better blood profile (lower cholesterol) and won't fuck up your metabolism.
That covers the supplement part. The rest is up to you - eat high quality, non-lean meat all the time, with lots of veggies and veggie juice.
And don't join a gym with the intention of "seeing how it will work out". Join the gym, find a good mass program, and work out like a fucking boss.