Mezue ahịrịokwu na-esonụ: N'oge asọmpi asọmpi nke 2010, a họpụtara Gallagher dị ka Nwoke Na-eto Eto Na-eme Egwuregwu nke Afọ na obodo ya Delta, British Columbia. Na atụmanya nke
2010 NHL Entry Draft, ọ nọ na 152nd n'etiti ndị North American skaters ruru eru ka NHL Central Scouting Bureau họrọ ya. A họpụtara Gallagher nke 147 site na Montreal Canadiens. Mgbe ọ laghachiri na nwata mgbe ọ gụsịrị ogige ọzụzụ mbụ ya na ndị Canadiens, o dekọtara WHL ọrụ kachasị elu na 2010<unk>11. N'abalị iri abụọ na asatọ nke ọnwa Febụwarị afọ 2011, a kpọrọ ya WHL Player nke Izu mgbe o nwetere akara iri (ihe mgbaru ọsọ ise na inye aka ise) n'egwuregwu atọ. N'ọnwa Machị afọ 2011, Gallagher nwere mmerụ ahụ n'isi, nke a na-enwe mgbe ọ nwetasịrị mmeri na egwuregwu megide Portland Winterhawks. N'ịbụ onye nke mbụ na ndị Giants na nke asatọ n'etiti ndị niile WHL na-eme ihe na 91 ihe karịrị egwuregwu 66, a kpọrọ ya na WHL West First All-Star Team. Ihe mgbaru ọsọ 44 ya dị naanị ihere anọ nke ndekọ otu egwuregwu otu oge nke Evander Kane.