The injury occurred in Scotland's home defeat to Wales at the weekend and means the 23-year-old is unlikely to return to action with his club until the first week of May. Gatland is due to name his Lions squad at the end of April and kick-off their trip with a game against the Barbarians at the Hong Kong Stadium on Saturday, 1 June.
Gray was carried off on a stretcher during the 28-18 defeat at Murrayfield and Scotland and Lions doctor, James Robson, confirmed the extent of the injury on Monday.
"The normal period of rehabilitation is in the region of eight weeks. We have kept his club medical team at Sale Sharks fully appraised of Richie's situation and will now commence his recovery with his club with a view to playing before the end of the season," said Robson
Gray's biggest problem is that his domestic season is due to end on the weekend of 4 May, when Sale Sharks host London Wasps in the last round of the Aviva Premiership, leaving him short of match practice if he is selected for the Lions tour.
Also out of the running is Irish scrum half Eoin Reddan, who broke his leg during Ireland's 13-13 draw in Dublin with France. Both Irish centres, Brian O'Driscoll and Luke Marshall, suffered concussion during that game and could be ruled out of their country's final championship assignment against Italy in Rome this weekend.
But all eyes in Round 5 of the 2013 RBS 6 Nations will be firmly fixed on Cardiff's Millennium Stadium, where Chris Robshaw's England side will be seeking a first Grand Slam since 2003. Six penalties from Toby Flood steered England to an 18-11 win over gallant Italy at Twickenham to make it four wins from four for Robshaw and co.
They will now take on the reigning champions, Wales, in their own back yard and will face a side running into some decent form after their opening day defeat to Ireland. Since then, Wales had won three games in a row on the road, adding Scotland's scalp to those of France and Italy, and have kept their line intact for 278 minutes.
Wales scored the only try among the four 'Home Nations' in Round 4, a bulldozing effort from hooker Richard Hibbard. Leigh Halfpenny kicked all the other points with seven penalties and a conversion.
The game at Murrayfield set a world record for the most penalty kicks at goal in an international, 18, and the 13 successes shared between Halfpenny (7) and Greig Laidlaw (6) were a 6 Nations record.