The Oldest Hero

Yes. You are wrong.
There are generally 3 ways to tackle down Gil
(1) Tamamo/Shishou Solo or team with Waver
(2) Karna/Later's NPBB critical burst before Gil's NP in team with Waver/Hans, or using David/Tristan to team evade Gils' first NP then slash out him.
(3) 3 Cus

Using Cu and support is somehow equal to 1+1<2 since supports will be easily slashed out by Gil as times goes by, and it is contradict to using Cu as a long tough fighting (lv 100 Cu still can not do comparative damage as Karna/Later/Shishou even you have dual Waver as support). However, Gil will accumulate his attack, then you may find that like >5 turns Gil can also deal very high damage to Cu if his evade is not on. Before that, your supports are extreme vulnerable and Gil could attack 3 times to eliminated Cu's evades in one turn if only one Cu in the battlefield.

3 Cus can distribute Gil's attack hit accounts to 3 evade status then eliminate this effect, and using Gil's attack as major source for NP gain. Do not use any support even Mashu or Leonidas as defensive supports.

Solo him with a proper Lancer support.
AoE Lancers are not ideal since their NP damage is already lower due to their AoE nature and even then, Gil's 10 turns, -50% NP debuff that cannot be cleansed.

Scathach and OG Cu are ideal here + Atlas Mystic Code.
The issue here is that you need to survive for long enough to whittle down his HP and he's pretty much going to hurt you a lot, even if you have the class advantage. That's why Evade, Invulnerability and Guts are king here.

There are some instances where people brought teams to fight him but my F2P ass is better suited to leaving a decked out support deal with him.

Search on YT "Gilgamesh VS X neforest". In particular, Honako Green has done a solo Cu run but his is level 100 (really loves Cu) and when Gil had a bug where he was reducing 99% of NP damage. People have already done Scathach solo and she can do it because her NP guarantees a stun.

You can use my Lv100 Tamamo Lancer NP3, she should be able to pretty much solo him down. 994,463,657