No, Grand Swell does not do more damage if Devoluter is in play; it looks for tools and stadiums, not technical machines.
Against Gengar, most of the time you're probably just going to have to do what every other straight-damage deck has to do: be extremely aggressive and take the Feinting Spell flips when they come. You can potentially use Uxie to get around the Power, but it will require good Super Scoop Up flips (on the active Gyarados) and is thus unreliable. Nidoqueen can also make this difficult. Crobat G is another option, and so is Skuntank G, although Nidoqueen is still a foil for both. You could also run a Switch or two to aid with the Uxie trick; that card would be good in other instances as well, like if you opened with Regice/Registeel. You could also run Pokeblower to nab you prizes on things aside from Gengar. By taking out a Claydol early on in this manner, you're likely to cut off a Gengar swarm, which lessens your burden if the few Gengar you do have to deal with end up taking you down with them.
If you're running Gyarados with Floatzel GL lv. X and minimal other rescue cards, this match-up will be more difficult because it will be harder to recover from every successful Feinting Spell. If you run 4 Pokemon Rescue and a handful of Night Maintenance/TSD/Palmer's, you should be able to rebound immediately and keep your offense going.