I never expect to be able to eat anything more (and that is if I am lucky!) than crudités (carrots, celery, perhaps broccoli, cucumbers or tomatoes arranged around a dip) when I am at a party, (and the dip is always questionable).
At first I would just go to parties hoping that there would be things that I could eat.  After several meals of crudités, (and a bit of pouting) I started taking a more proactive position.   I am not comfortable telling acquaintances about ways they might enable me to nosh through more things at these events.  Even with close friends and family, there seems to be this haze that envelopes their heads when I try to explain “no flour, bread crumbs, bread or pasta” (although in truth it is a lot more complicated than that), sometimes I feel as if I am speaking a different language.  I have shown up at holiday meals, with family members saying to me when I asked “What is in the mashed potatoes?” …“Oh, Lia, they have butter in them, I forgot!”  “I guess you can’t eat it”.   Really?  What part of butter has flour or wheat in it?  I give up.
In self defense, I have taken to bringing things with me.   This also eliminates pressure from the host or hostess from trying to accommodate my “strange unfathomable” diet.  (I am sorry did I say that out loud?)
Things to bring with you to picnics or an outdoor grill party; See my Product Review for specific suggestions.
Gluten free hotdogs, or your own GF deli meats.
A couple of slices of your favorite GF bread (toasted), remember it tastes so much better this way, and since it travels better toasted rather than “raw”, it is a win win scenario.
GF chips, there are plenty of corn chips on the market or rice cakes.
GF Crackers are a good thing to bring, in case cheese is available.

For dinner parties, I often eat before I arrive; does this sound radical to you?  It ensures me of having a pleasing meal, and not feeling frustrated at people eating around me, especially if I cannot.
Remember it is about taking care of you, physically and emotionally.  Do what ends up leaving you happy.
The only time this backfires is if someone goes out of their way to prepare a GF dish, and in my experience, this does not happen often.  In these cases I will eat a little of what they have made (not being very hungry because I am full).
Birthdays:  I always bring my own piece of GF cake, (if I do not have time to bake one for the group).  I keep slices of GF cake, cupcakes and or brownies, individually, triple wrapped, in the refrigerator for just such occasions.   I don’t like feeling left out.  Bringing my own, allows me to make sure I can be included.