The earlier time(s) he survived Oozaru Goku, Goku could have been much weaker. Say Goku's PL was only 4 or lower. That's low enough for Gohan to handle it easily, but enough to recognise that Goku had become absurdly large and strong while transformed. The last time Goku's PL could've been 8-10. If Gohan isn't careful enough, he could be hit and then stepped on. Say if he's trying to stop Goku from walking off a cliff, but without hurting him and then gets hit.
As for Goku claiming he never looks at the full moon, does Goku remember looking at the moon after he transforms (against Pilaf and Roshi)? If not, he could've forgotten about the one time he looked at it. Plus even if in cases of transformation he can usually remember seeing the full moon, if it was while he briefly woke up at night, he may not remember looking at it.
As for lack of house damage, yeh he could have woken up while sleeping outside and seen the moon. It's unlikely Gohan would have Goku sleep outside a full moon night though. They could have been walking on a full moon night and he saw the moon's reflection in a pond. Maybe that's enough to transform him? But going for a walk on that night is still a big risk for Gohan to take. Maybe it was very cloudy and he didn't realise it was a full moon. Goku goes to a pond, the clouds part and he sees the reflection.
Another possibility is Gohan went outside for something. He usually tells Goku when it's a full moon night, so Goku won't look up on that night, for fear of the monster. He went to a village planning to come back before the full moon night so didn't tell Goku the schedule, but then Gohan gets held up. Goku went to sleep as usual, but woke up and then saw the moon. He's far enough from the house that he doesn't destroy it and starts rampaging elsewhere. Gohan gets home and gets killed.
Does Goku know that the monster who killed GG is the full moon monster? Maybe he follows the full moon rule more strictly only after GG's death?
PS just seen this comment on an article. The hell?
