Should I power up my Donphan?

i dont see any reason to power it up (imo)

You need more candies to bring it up to a more appropriate level. Wait until you can bring it to 30 at least

I would wait on any questionable mons until the gym fix is made. The only mons I ever see in gyms are Snorlax, Vaporeon, Dragonite, Blissey, Tyranitar, Rhydon and Gyarados. Golem and and Exxegutors arent around anymore and Vaporeons ate rare too.