Change your preferences here.
When this Pokemon is hit by a super effective Electric-type move, the damage is reduced by 50%. This item is consumed during the damage calculation. If this item is recovered via Recycle, it can activate again. Provides a one-time 60 Base Power Electric-type attack when a Pokemon holding this item uses Natural Gift.
Type-resist Berries are used to allow a Pokemon to survive through a common weakness while it sets up or KOes in return. Not many Pokemon have a 4x weakness to Electric that stops them cold, but Gyarados is one. It can use the free turn while the Electric attack is consumed to obtain another boost.
Wacan Berry is seldom seen, so the opponent will be taken surprise and fail to play around it. It gives Gyarados a free turn, allowing it to boost its Attack and Speed to further heights. It also enables it to take on walls like Vaporeon better.
However, Gyarados with Wacan Berry suffers from several concerns. The first is that without Life Orb, its damage output often simply isn't high enough; +2 Adamant Gyarados does only 51.12% - 60.27% to 188 HP / 252 Defense Bold Vaporeon with Stone Edge, so it will fall afterwards. The second is that Wacan Berry is one-use, and even while holding it, after taking damage from Stealth Rock, sandstorm, and other resisted hits while coming in, it will still be KOed by many decently strong Electric attacks. The third is that Electric is not its only weakness, so it is still very vulnerable to Rock-type attacks, and any neutral hard-hitting attack, such as Celebi's Grass Knot.
Wacan Berry should not be gratuitously given to Pokemon. It could perhaps see niche use on Pokemon such as Mantine, but in the end, for walls, Leftovers will generally be a superior option. Keep in mind that most Electric attacks are special, and Gyarados in particular has very good Special Defense.