The Oxfordshire official allowed the Italians, who have already beaten Leicester City, Crystal Palace and Norwich, to get away with some strong-arm tactics which saw strikers Lomana LuaLua and Ivica Mornar go off with injuries. And to cap it all Gallagher awarded the Serie A side a controversial penalty.
Portsmouth had the best chances of the first half and LuaLua was unlucky to have a header ruled offside in the 18th minute. Despite his early exit from the action, LuaLua was given a clean bill of health by the Portsmouth coach, Joe Jordan, following the match.
Brazilian Adriano, who scored 28 goals for Inter last season, should have netted with a close-range header that he put wide before impressive new Colombian midfielder John Viafara had a 25-yard blast deflected wide and Laurent Robert curled a free-kick just over.
Gary O'Neil nudged a great chance over the bar at the start of the second half after good work from Robert and Norwegian striker Azar Karadas.
But Inter took the lead from a disputed penalty in the 54th minute, with David Pizarro beating Sander Westerveld from the spot after the goalkeeper was harshly ruled to have pulled down Obafemi Martins.
Vincent Pericard, who has missed most of the last two seasons with cruciate knee ligament damage, deserved an equaliser after coming on as a half-time substitute.
But the Frenchman drilled his shot just wide after a superb link-up with Robert and O'Neil in the 70th minute.
But Pompey were caught flat-footed six minutes later for Dejan Stankovic to slot in Inter's second.
At Sincil Bank a first-half strike by Andy Parkinson gave Grimsby their first friendly win of the season. In front of 1,523 fans in Lincoln, Parkinson rifled home from 20 yards to put the Mariners in front.