User blog comment:Warmachine375/When will Gun Gale Online become an anime?/@comment-71.89.82.47-20130801131510/@comment-6697950-20130811052446

A season is considered a batch of episodes that are ordered in one go. If there isn't any kind of break in-between the arcs, then they are both part of the same season.

The reason why they changed the OP and ED is obvious - it would be illogical to still use the 1st half's one for the 2nd half, since they changed the setting to another game.