an older usage for designating the interior of a church; originally, the Gospel Side was the north side [the left side facing the altar]. See Epistle Side.

when looking into a church fromt he back it would be the left hand side. Traditionally this has been the side where the Gospel was read from and where the pulpit stood.

The side of the altar at which the Gospel is read, the left as the congregation sees it. Traditionally the side at which the pulpit is located.