frostmephit ability: "An enemy unit that targets FrostMephit with any attack or ability is frozen for 1 turn.
Immune to Freeze."
behemoth ability: "Cannot be targeted by enemy minions."

gravitymage is frozen even though it targeted an empty location. i guess this implies that the teleported unit is targeted.

since it should be a target by frostmephit-gravitymage interaction, not sure why nexus can teleport behemoth.