Been trying and trying he’s not really appearing anywhere for me
At this point of the campaign (assuming its for the campaign), your best bet is to get lucky refreshing/finding him in casual pvp or keep running Explore Mythic Battles in Grosh-Nak (where the Mythic Boss can either be Gargantaur or Shade of Zorn).
If you find a Gar’Nok, you might be able to get him to summon a Gargantuar
If you own a The Possessed King or Krampus, you can try using its spell and see if a Gargantuar gets transformed into battle.
When you get to Grosh-Nak mythic battle, if it isn’t Gangantaur (free scouting or pay 50 gold), go back (your mythstones will be refunded) and exit the game completely. On Android phone, swipe to kill the game.
Reload and it will roll a different team.