With Gerson's comment about Dragon Blazers being its own interpretation of Lord of the Hammer that takes its own liberties, I think Gaster isn't meant to represent a "game developer" but a "fan-game developer".
There once existed, an original timeline, with no prophecy or dark worlds, it was just the Light World and characters going through mundane issues until a tragic event happened. It wasn't even a game just its own reality that began and ended.
"Deltarune" is Gaster's creation but what exactly is it? It's a reboot of the world with fantasy elements getting tossed, a straightforward "save the world" plot, and it's a video game where (you) go in to progress the story and fix the characters' problems. No doubt it's a story about "defying fate" but really you're not the one doing it. Gaster's the one trying to defy fate via rewriting reality and dragging you along to experience it.
He doesn't expect much from you, just to follow the plot and the prophecy will be avoided in linear scripted events which he has written out.
But the player can still throw him off-guard. We can win a fight intended to be a scripted loss or get 0 console achievements which has him shocked and excited for what else we can accomplish.
Perhaps Gaster also wants us to take the future into our hands, and he's willing to let us take the reigns if the outcome is better than the one he planned out.
(Though through something like the Snowgrave route, we are forcing the game to become a transgressive horror story about abusing a girl, possibly bringing the story back around to the initial timeline/prophecy and ruining Deltarune)
Deltarune is a game about creating things based on pre-existing things. It's about the fan's relationship with art and its creator. It's about passing the torch, allowing people to take your creations and make their own things with it. What is "creating a new future" when a story is already finished and the ending's set in stone? Fanfiction!