In our area, from a kid-to-graduate perspective, gift giving only happens amongst the closest of friends. And - from the friends that my DD got gifts from - they were well thought out gifts, not just a GC.
If you do feel the need to give something - a GC for the movies is a great idea, or a GC for gas. If you are more of a present person - a really inexpensive idea is a cheap tool kit that they can take to college - can be found for about $10 or so.
Seriously - it isn't necessary. Put this in the context of next year when he graduates, and will be invited to a hundred parties. You will be glad that you don't need to buy gifts for a hundred grads.
Parents don't get off that easy - if they get invited, it is appropriate to bring a gift, in our area, it is typically around $25 for a casual friend of the family, and we usually do cash, as that is what is needed the most. Goes up from their for closer friends and more for nieces and nephews. A fun gift to give is a check that represents the year - i.e. $20.14. Next year, we will have a couple of graduation parties, and I am going to be making cards that have a $20, $1 and a $5 bill in it, with the bills folded so that it spells out 2015.