My dad used to do it. After my mom died, every single time he came over, he had gifts. Of course I didn't say anything because for whatever reason he wanted to do it. Then one day when he pulled up my kids ran outside to greet him and asked him what he brought for them. After he left that day I had a pretty serious talk with them about how they should never expect a gift and that when they do that, grandpa might feel like they are happy to see him because they want a toy, and not just happy to see him for him. They seemed to get it and have never done that again.
I also did mention to my dad that day not to feel like he always has to bring stuff for them. That he is enough for us. It was kind of a tearful exchange to be honest. I know what he was trying to do.

He's a freaking saint and the sweetest man I know. Anyway, he eased up on that and now he will call before he comes and ask if I need fruit (lol, sounds strange but he lives near the co-op I like to buy produce from) and then he brings every single sort of fruit you could ever imagine.
My mil always brings gifts but we only see her twice a year.
I do think it's more important for us to teach our children about gratefulness than it is to try and stop people from giving them gifts. I really feel like a lot more goes into a "spoiled child" than just frequent gifts.