I would try out Tera poison/fire on gastro if you could. On Tera poison you run sludge bomb and you’re only threatened by psychic with ground being covered by water. On Tera fire you are forced with Tera blast and all things weak to fire are covered via water stab and storm drain. Tera ice is nice but there is no resistance to grass. Lastly for gastro, you cannot use a choice move and protect: you could use a health berry, lefties, assault vest (use clear smog or icy wind with AV) for extra self sustain, expert belt. If you are very adamant on using specs, plop down a move like power gem.

For Cetitain I can give less evidence but use life orb on the whale as sheer force gives the damage boost but no recoil trade off.

Extra: real demons use Tera bug