The 41-man squad and all the coaching and backup staff convened at a west London hotel to go through various administrative tasks and collect their mountain of kit ahead of the daunting 10-game tour of New Zealand in June and July.
Among them were Scotland scrumhalf Greig Laidlaw, who was called up over the weekend following the withdrawal of England's Ben Youngs after his brother Tom's wife was diagnosed with terminal cancer.
Laidlaw and Welsh backrower Ross Moriarty will play for Gloucester in the European Challenge Cup final against Stade Francais on Friday, while six more Lions are in action for Saracens in the European Cup final against Clermont Auvergne the following day.
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Saracens cancelled their planned Monday training session and the London club's director of rugby Mark McCall said the timing of the Lions administration day was "unbelievable".
"I understand Mark's reasoning but if it had been last week it would have potentially affected 22 clubs," Gatland said.
"I understand it's a massive week for them but Mondays tend to be a recovery day. This date has been communicated many months ago - I don't know when Saracens got the information and we haven't had any requests from anyone to move this date.
"But we're very grateful to Saracens and Gloucester for releasing their players. It does really make a big difference. It's exciting, but also a very important day for us."
Gatland, who will be crossing his fingers that he does not lose any players to injury next weekend or in the subsequent playoffs and domestic finals with the opening tour game looming on June 3, said there was a good atmosphere at the first group meet-up.
"It's kind of like the first day of school, they are a bit nervous and apprehensive, but they're introducing themselves so it's pretty exciting," said the New Zealander.
Gatland said this was not the time for any big motivational speeches but the coaches will be addressing the players about the expectations that come with being a Lion and the particular challenge of touring New Zealand.
"We've got plenty of preparation ahead of us," he said. "But all the heads of department will start getting their messages across today."
(Editing by Ed Osmond)
