Gaster doesn't really fight as he's tugged to the garden table, more or less resigned to have this conversation that he had been so desperately avoiding. It's not worth hurting Asgore over this; not when part of his purpose is to spare Asgore of having to know him in the first place, of caring about him in the case that what he did comes out.
Frisk appearing at his side right after Asgore left is a surprise, but it perhaps shouldn't have been.
He's long figured out what sign language the kid was using, so when they engage in idle chit chat with him, and he can respond accordingly.
He's a little disturbed by the kid, if he is being entirely honest. He's not even sure why.
Or well, not sure entirely why. The uncanny way they resemble Chara is definitely part of it. The same brown hair at the same length, similar facial features, similar heights, and even similar ways of dressing, though with a different favorite color pallet.
Is this... good for Asgore? To share custody of a child that so resembles his dead child?
Something else about Frisk reminds Gaster of Chara- not their SOUL color- but thinking about that too much makes his head spin, so he puts it aside.
And then Asgore gets back with the tea stuff and sends Frisk on their way.
He doesn't contradict as Asgore talks about how they must have been close friends, just ducking his head.
❝TRY TO CONSIDER THIS FROM MY PERSPECTIVE, SIR,❞ he says weakly to the first question, though when asked if Asgore offended him he looks up, fixing his gaze on him.
❝ASGORE, NO, NO-❞ he shakes his head quickly, ❝YOU DID NOTHING WRONG! AS- AS I WAS JUST SAYING, TRY TO SEE THIS FROM MY PERSPECTIVE. IMAGINE, JUST IMAGINE HOW YOU WOULD FEEL IF ONE DAY, ABSOLUTELY EVERYONE YOU'VE EVER KNOWN COMPLETELY FORGOT YOU. IMAGINE-❞ he takes a shaky breath, ❝IMAGINE YOU HAD TO INTRODUCE YOURSELF TO GERSON, AFTER ALL THE YEARS YOU'VE KNOW HIM. IMAGINE HOW MUCH IT WOULD HURT FOR HIM TO NOT ALREADY KNOW YOU INTIMATELY.❞
It was a lot to say at once, and it's not the full reason, but he hopes it will get his intentions across to Asgore. That it has nothing to do with anything Asgore did.
❝... I WAS A MEDIC,❞ he says softly, ❝A DOCTOR, LONG BEFORE I WAS A SCIENTIST. THAT'S WHAT I'M DOING NOW, ACTUALLY...❞
Finding the tattered remains of his old medics coat from the war- carefully persevered and stored away- while Papyrus let him go through their stuff to find old belongings of his sent him into uncontrollable sobs. Which just made Papyrus start crying- he's so empathetic- and Gaster had to leave immediately. He almost definitely still has things there (not counting all the stuff he was just going to let them keep), but it seems like his past is too sore of a subject still to manage that task. He can't be that vulnerable in front of Papyrus again.