How about the traveling factor (making it at home and taking to someone's home) and reheating it. Any suggestions. And thank you

You can make it a few hours ahead if you'd like. Just undercook the gratin by about 10 minutes and cover it to transport. Uncover and put in the oven to finish cooking just before serving.

Someone once said to me that potatoes au gratin was like napalm. It seems to retain heat like for a long time. I would suggest wrapping the hot dish in a layer of aluminum foil, then wrap the pan with a thick bath/beach towel or two. Then put this bundle into either a cooler or any insulated bag (those chill bags for shopping are great). It should still be warm by the dinner time and probably not need any reheating.

I'd say it'll be fine. You could either cook it fully and then reheat, or cook it a bit shy of the full time so it finishes off at the destination. Will you be cooking in advance and reheating from chilled, or cooking it just before you travel? If cooking in advance, I might add in a bit of extra wet ingredients to things moist. My general rule of thumb is that if the supermarket sells it in the chiller cabinet, it'll work well to reheat a home made one....