The attacking batsman will turn out for Royal Challengers Bangalore, owned by Indian billionaire Vijay Mallya, as a replacement for the injured Dirk Nannes.
Gayle had been under a treatment programme run by the West Indies Cricket Board (WICB) after a strain suffered during the World Cup, but he signed up for the Bangalore outfit on Tuesday for the remainder of the IPL event.
Gayle had earlier refused a West Indies' central contract but was still in contention for games during Pakistan's tour, which starts on Thursday.
He was not bought at the IPL player auction in January because franchises feared he would be unavailable because the IPL clashed with the Pakistan tour.
"It's official now. The explosive Chris Gayle is now a part of the RCB squad," Mallya wrote on his Twitter page.
Pakistan will play one Twenty20, five ODIs and two Tests against West Indies during their tour.
The WICB said it had reluctantly cleared Gayle, a former national captain, to play in the IPL but that it was "most disappointed" at his conduct.
"The WICB does not accept Gayle's reason for making himself unavailable but as he has already clearly made his choice and has travelled to India, the WICB does not wish to stand in his way," it said in a press release.
Gayle played for the Kolkata Knight Riders in the first three editions of the IPL.
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