If The Gospel According to Heather were billed as theatre for young adults, tween crowds might find it appealing, but it is likely to land as trite and trivial with adult audiences. As TYA, though, the show at Theatre 555 might need just a few edits. Like when popular kid Kaisley (Maya Lagerstam) advises Heather (Brittany Nicole Williams), "You're in high school. You're not supposed to find yourself. You're supposed …