Gatland selected a 37-man party for the current Australia tour, but props Cian Healy and Gethin Jenkins have already gone home injured, wing Tommy Bowe is recovering from a broken hand and full-back Rob Kearney is involved for the first time this weekend after being sidelined by hamstring trouble.
Alex Corbisiero, Ryan Grant and Simon Zebo have joined the squad, while centre Billy Twelvetrees is currently on his way from Argentina. Saturday's appointment with New South Wales Waratahs sees the tour reach only its halfway point. "I think that as you move from one tour to the next, you realise how tough it is, especially for the players," Gatland said.
"The game's physicality has increased from 2009 (Lions tour) in South Africa. It has stepped up a level, and players handling two games in a week is tough. It is something we need to be mindful of.
"If you are writing a report at the end of the tour, you need to consider whether there is a case for bigger squads from the start because of this step-up in physicality."
Gatland revealed that injuries could play a part when it comes to selection for next week's first Test against Australia in Brisbane, and the Lions still have two games left before then. Wing George North and centres Brian O'Driscoll and Manu Tuilagi are among those not involved against the Waratahs.
"I think there are one or two that are a little bit sore that may next week be out of contention because they won't have been fit enough early enough in the (Test) week," Gatland added, without naming specific players.
"That is potentially what might happen. By the time Saturday (the first Test) comes round, they might all be fit, but one or two are possibly going to be ruled out because they are not fit enough earlier in the week.
"Manu has a sore shoulder, but it is improving all the time and he is training. George North's hamstring is a bit tight from the other night. We've got a few bodies among the backs who have been knocked around, which is why Billy Twelvetrees has been brought in as cover.
"Jonny Sexton and Owen Farrell have both been passed fit by the medical staff. We could easily have gone again with Stuart Hogg at 10 (this weekend), but Jonny is playing."