The Lions, who will arrive in the country on Wednesday, May 31, four days before their first match against the New Zealand Provincial Barbarians in Whangarei on June 3, also face the Blues, Crusaders, Chiefs and New Zealand Maori before the first test at Eden Park on June 24. Their next match is against the Hurricanes before the tour finishes with two more tests.
Gatland will announce his squad in London tomorrow and Henry said while the 37 players will be thrilled to make the tour, all of the fixtures will be extremely challenging.
"I know from my own experiences how much the players respect getting selected for the Lions," Henry told ESPN. "It is the pinnacle of their career. It is massive but they need to do well and I just wonder if the itinerary is suicidal. That is my concern.
"They are playing New Zealand Maori, they are playing the five franchise teams - and those five franchise teams have nothing to lose, no pressure on them at all, so they will fire everything at the Lions and take them on.
"Hopefully they [the Lions] have the ability to overcome that. But really when you tour, you need to ensure some momentum is created by results and you just wonder how they are going to go into the test series with that itinerary. It is very demanding."
Henry coached the Lions on the tour of Australia in 2001, a 2-1 series defeat, and 10 years later mentored the All Blacks to their second World Cup victory.
The Lions haven't won a tour of New Zealand since 1971, and a hint of what they will face this time was given last year when the Chiefs humiliated Gatland's Wales team 40-7 in Hamilton.
"The Chiefs had nothing to worry about, no pressure, they just went out and expressed themselves," Henry said. "When those young kids do that in this country, against opposition that they respect, and they are obviously going to respect the Lions, it is going to be a massive challenge for the Lions."
Henry added of the Lions: "They are remembered by the test match results but sides gain confidence and momentum through the games they play leading into those test matches and if they don't get success in those games, confidence is not going to be high and that is going to affect the way they play in the test matches.
"I don't know if you can draw a line between the games you play prior to the test and the test.
"I think it is all part of what you're doing there and part of the psyche of the team, the confidence of the team and how they play.
"Certainly they are going to be remembered for how they play in those three tests, but the building of the confidence and the ability to play the game through those franchise games and the New Zealand Maori game will be pretty important."