You don’t get it. The simple fact you have a pet out means that Revive Pet no longer exists in WoW’s spell DB for your character while your pet is alive. If your pet is out and you hit save all Revive Pets will be automatically be replace (not by GSE but by WoW) with Mend Pet. If you dismiss your pet it then goes back to Revive Pet.
Now once combat starts GSE can no longer change your macro so if it’s Mend Pet - guess what - it won’t res your pet. If however it is Revive Pet (because of the way base spells work) then it will both res your pet or heal it.
To trick WoW - dismiss your pet first then when you edit do the following (including the $$ at the front)
$$/cast [nopet, nodead] Call Pet 1; [@pet, dead] Revive Pet; [@pet, nodead, nochanneling] Mend Pet
Oh and read up on how Base Spells work: Two most asked questions: Base Spells and Macro locks