User blog comment:CNBA3/Fairy Tail Civil War!/@comment-24546844-20160406115347/@comment-25111584-20160406131211

Loke already chose Gray over Lucy in Tenrou Island arc. Sure, it was a decision made far before he ever met Lucy, but he didn't seem to have any qualms about the fact that he might have to face off against her at some point.

Purely hypothetically, if Loke had to choose between Gray and Lucy, his decision will most probably depend on the situation rather than the people involved. Of course, assuming that neither side's case is blatantly false.