They will be joined by Chennai Super Kings and Mumbai Indians (both India), South Australia (Australia), Warriors and Highveld Lions (both South Africa) and Central Districts (New Zealand).
Gatland said Wales captain Ryan Jones was one of the leading candidates to lead the Lions during their six-week trip, which starts against a Highveld XV in Rustenburg on 30 May.
The Wanderers ground at Johannesburg attracted the highest crowd for a game between Highveld Lions and Nashua Dolphins which saw Lance Klusener and Shaun Pollock steer the latter to victory with one ball to spare.
He enjoys being able to add another "12 to 15m" in length to his kicks in the higher altitude of South Africa's Highveld, where the Lions will return for the second and third Tests in Pretoria and Johannesburg respectively.