This is strictly with neutral enemies, servants being level 80 and 1000 fou'd, all skill activated and talking about NP5.
Salter already has way higher np damage than Siegfried or Gawain.
Under normal circumstances Siegfried Deals 28507 while gawain 2544.
By sun lit stages Gawain deals 33073 thanks to the buster up buff when condition meet (last 3 turn)
Dragon type enemies take 76969 from siegfried but his buster buff last 1 turn but it isn't tied to anything.
As you can see Gawain barely outdamages Siegfried.
Gawain do have a np charge skill which charges 20% np there is that. But with his upgrades Siegfried after using his np gets NP gain buff so he can also recharge pretty fast.
I won't say to burn him because maybe one day he will be potentially usefull, but if u don't need an aoe saber right now, then you can skip him now.
CCC will have him as an event bonus servant, but there aren't that many Lancer mob or if there are they are usually 1 per round with slightly high HP so you need Single target saber against them rather than AOE like Gawain