We've eaten at Grand Floridian Cafe many times, and most have been without an ADR. My youngest DD loves to eat here - she loves the Mickey shaped cheese & grapes appetizer on the kids menu! We find it to be a very relaxed, casual meal away from the hustle/bustle of the parks! (We've also had breakfast here twice, without an ADR, but it was at a later time - maybe 9:30 AM or so.)
I did make an ADR for our June trip for lunch, just because I was already on the phone making our ADRs. I think you'll be fine without one though.
Enjoy!!!!