Pikachu's bulk is 35/30/40
I'm not suprised you Pikachu is getting wrecked, it's defense isn't going to let it tank any kind of hits apart from weak, resisted ones. My advice would be to EV train in Def/Sp.Def if you absolutely have to, use a Focus Sash or just power level till you can outspeed and KO Gyarados
Levelling increases stats, and EV training in certain areas helps increase the stats of one more. However due to Pikachu's low base bulk, his def isn't going to go much anyway even through EV training, but if you have to train against things like Aggron for Defense and high SP.Def Pokemon for Sp.Def EVS