I'd say that Emolga would be better suited for Skyla's gym, especially if you're able to trade your Boldore for a Gigalith. Emolga can outspeed most, if not, all of Skyla's Pokemon.
And Swanna is 4x weak to Electric, and Gigalith would get outspeed and probably die from Surf.
Even if you can't trade your Boldore into a Gigalith, I'd still say that Emolga is better for the reasons I've stated.