In the void Underfell Gaster was taking a nap. He had his eyes closed, had a tooth pick in his mouth, floating, laid back with his arms crossed, he had sharp teeth like every monster in underfell.

(I use BunnyMuse's UfGaster)

The snowstorm ended, Niko was floating in the air, guiding Red to the secret door to the void in Waterfall. Niko preferred to float.

Red: you sure he's around here? I come here everyday.

Niko: *nods* y-yes. I have been in my own void for years, in there, I have talked to him many times before..

Red: wait, so you actually know the guy?

Niko: he's like my older brother I never got to have. He is grumpy, a bit rude, but he has a good heart.

Niko showed him the secret door. Red inhaled, preparing himself. He was about to meet his creator, basically his father. He wasn't sure what to say, what to expect. He opened the door slowly. Gaster opened his eye.

Gaster: ..hmm…? The hell- sans..!

Red:..d-dad?

Gaster floated down, ran to red, hugged him tight. Red hugged back

Gaster: Sans!..we finally meet again..! H-how!?

Niko: I lead him to the door

Gaster: wait, Niko, you're free!? How is this possible!? How did you escape!?

Niko: the original Gaster did. He's in search of any trapped Gaster, is rescuing them from their voids. I'll contact him right away.

A portal opened, the original Gaster stepped out

Gaster: no need, Niko. I'm here now.

Ufgaster: ..Undertale Gaster right..?

Gaster: that is correct, my boy. Niko, Sans, come. Let us go. I can take it from here. Wait at home.

Niko: w-wait! Gatsy! I-is he-!?

Gaster: *smiles gently* he is okay. His soul was poisoned, he is resting in my lab, its not bad enough to kill him.

Red: woah, woah, who's soul got poisoned now!? Jeez! So much things going on at one time, its really damn hard to keep up with!

Niko: I'm in a relationship with another Gaster..I call him Gatsy..He never fell in his void, is a full time father to his Sans and Papyrus..we were exploring Aus, we came upon HorrorTale, that au's chara poisoned Gatsy's soul, possessed him, attacked me. ..I escaped to this world, seeking for help..

Red: thank you. That's much easier to understand.

Gaster: alphys is with him, working up a cure for the poison.

Niko: thank you *smiles*

Gaster: now, go. I must begin my work to free this Gaster now. This will take awhile.

Red: I'll go with you.

Hours later, papyrus was home, ironing clothes, the door came open. Red called out

Red: Hey, paps! I brought you someone you will want to meet!

Papyrus turned, looked. Ufgaster hugged him tight. Papyrus froze

Papyrus: ..d-dad..!?..

Ufgaster: you and sans have grown up so much..I'm so proud of you ..

Red: welcome back, G.