The Player going first in the game cannot use a Rare Candy as they are not allowed to play Supporters/Trainers on their first turn.
The Player going Second however may use Supporters and Trainers, so they may use a Rare Candy to evolve their Pokemon. But, if they use a Rare Candy on a Pokemon, they can't evolve that same Pokemon again.
So this would NOT be allowed:
Gabite > Rare Candy to Gabite > Evolve manually to Garchomp
When it says "This counts as evolving that Pokemon", this means that that evolution counts as your evolution for that Pokemon for that turn. (That's a lot of "that's" lol) So you may not be able to evolve a Pokemon twice in 1 turn.