This one is tougher to answer because I DMed the series so many times that they all run together. I believe I bought all 3 original modules of the G series (before they were repackaged as a unit) back in 1981. Some of the runs through it were totally munchkin, as the party had so many magic items that the giants were no challenge. The worst case of this was the time that the party came into the series with a Hammer of Thunderbolts, then found the one in the module too. 2 was
way too many. :\ High level Rangers with Giant Slayer swords were problematic also.
The players tended to treat the giant lairs like any other dungeon crawl - their plan was to move room by room through the place Terminator style, killing everything in their sight & looting anything not nailed down. As my DMing skills increased this became harder & harder for them, so these modules were a touchstone in my development as a DM. The lessons I learned here still prove useful 20 years later.
Probably my most distinct memory of the series was the near-TPK. One of the characters was the (seemingly inevitable in 1E) Assassin-pretending-to-be-a-Thief. While I disliked Evil characters in general & Assassins in particular, since they were legal in the PHB I didn't feel I could ban them. DMs who arbitrarily banned classes & ethos were frowned upon back in those dark days.

Anyway, the party invaded the Fire Giant lair & after some early success began to get hard-pressed as the alert spread was given to the giants of the complex. The party holed up in a room, the party fighters holding the doorway against the increasing press of giants. As the pile of giant bodies grew higher & the party's magical resources dwindled, the adventurers realized that they might have bitten off more than they could chew. The decision was made to break out of the room they were in & exit the giant lair as quickly as possible, before the party got overwhelmed, to live to fight another day.
And at that moment one of those unforgettable D&D moments occurred - the Assassin backstabbed the Paladin.

Unfortunately for the (now visible) Assassin, his assassination attempt failed. The Paladin survived & turned to face this new threat from within. While the party made short work of their erstwhile ally, the distraction allowed the giants to regroup & overwhelm the party. Only the Cleric, who quaffed a potion of Shapechange & flew away as a bird, survived the encounter. While that was bad enough, as DM I had a bunch of angry players ready to lynch the player of the Assassin - & me for allowing this to happen.

When asked for an explanation, the offending player said that he figured the party was doomed anyway. By backstabbing the party he expected the (Evil) giants to embrace him as an ally, so at least he would survive.

I pointed out it was rather unlikely that the fire giants would trust a stunty human who had backstabbed his own teammates, especially since he had done nothing up to this point to earn the giants' trust. This line of reasoning was met by the player with a blank stare. Ah, good times!
