You do not need to check-in online as you'll be there right when check-in opens. One upside is they open up more seats for selection at check-in but you'll also be able to request a seat at the check-in desk. The desk is manned by Menzies Aviation staff like many other airlines at LGW.
Advantages of the lounge are it's a quiet, calm place to hang out with generally complimentary food, drink & wifi instead of paying for each item. The views can be quite nice for plane watching also depending on the lounge. You can read reviews on such sites at loungebuddy.com I personally avoid the No. 1 North terminal lounge after a poor experience with the staff.
Nothing wrong with asking questions.