GF is hard, most people have no idea what contains gluten. I have to eat GF myself and trust me my relatives don't get it. I come from a large Italian family and they will say stuff like, "well can't you just eat a little bit of pasta?" Ugh! Yeah I could if I want to have a skin rash and explosive diarrhea--thanks, but no thanks!
Here's the deal: Your son is 2. Until he old enough to moderate his own diet, you will have to do it as his mom (or dad), there is no other way. Do not rely on others to check labels or make GF foods for your son. Even if you were to give them a list, it would be a couple pages long and they most likely would not follow it very well. Do what I do for myself--pack his food with you when he goes by their houses. Be firm that he is to eat nothing that you did not bring for him. Nothing! That is easy enough to follow. Make sure to bring some GF treats that they can give him. As for toys, Play-doh is about the only thing I can think of that he can't play with, so I think that should be fairly easy to request.
If the relatives do not comply with your wishes, then you will have to refrain from taking him over there or him having visits without you there. This issue is not up for discussion or negotiation and you need to make sure they understand that. This is your child's health we're talking about--nothing is more important. This is one thing you should be making a big deal about--either they comply or they don't spend time with your son. Don't make your son suffer for their bad judgement.
You need to be assertive on this issue. Explain exactly how things will be handled and what will happen if they break your rules and then follow through. Stop having discussions about this--it is not negotiable!