Gaius is only stronger than Bedivere if you consider him in a vacuum and forget that Merlin exists, at least until Skadi gets released. Also, even if Bedivere does slightly less damage by self buffs, Bedivere has a 30% instant NP gauge skill, which is more than enough to make taking him more useful than Gaius.
NP1 Bedivere = [(7627+2000+1000) * 8 * 1.5 * (1+.3) * 0.23 * (1+.15) * (1+.16)] = 50864 average neutral class and attribute damage
NP5 Gaius = [(7497+2000+1000) * 20 * 0.8 * (1+.08) * 0.23 * (1+.16+.15) * (1+.18)+125] = 64614 average neutral class and attribute damage