This is a really hard decision. But on views of battling, G-max
Eevee would be more powerful than Sylveon. While, Eevee into
Sylveon will only take a need to be sent into a rumble to be
evolved whilst Eevee into G-max Eevee will charge 4 Dynamax
Crystals. But, Sylveon will also help in getting a dex entry too if
you want to complete your Kalos dex to get its certificate.
It would depend on what you value more. If you battle often, then
Id recommend Giga-maxing it. If you value Dex completion, then Id
recommend evolving it.
My suggestion is that you focus on completing dexes since you're
early in the game, so I personally recommend that you evolve it. 😊