Gideon's first appearance in The Purifying Fire features him using cool heiromancy to nullify Chandra's magic. Then his origin story in, uh, Origins, tells of how he first learned heiromancy back in Theros before his spark ignited. We see him using cool and creative spells to knock his enemies off balance. Even in the Battle for Zendikar, I think he used some heiromancy here and there. The Battle of Fort Keff seems to imply that he used his magic to get all the enemies to focus on him.
After that, we see him use little to no heiromancy at all. All we see him use is his own natural invulnerability. Even his cool sural goes away.

So if he has all of these cool magical skills, why doesn't he use them in the later stories?

Lazy writing, aka Creative losing interest in serious storytelling.

I assume when he was put on the chopping block someone decided it wasn't important to keep things together.

That might factor into it as well, but on the other hand, it's not like they did a great job with keeping things together for other characters. They ignored that Jace used to summon tangible creatures in AoA for example, so they could pretend he was useless against physical threats like werewolves or Eldrazi. They ignored previously established rules about how a gorgon's stone gaze works when Vraska started interacting with other characters more. They ignored the fact that Bolas's mere presence was enough to shatter planes before he lost his Oldwalker powers. Just from the top of my head. And that's just talking about characters' power sets, I'm not even going to go into how they mucked up the continuity related stuff, or the worldbuilding stuff, or, like, basic logic and common sense in general.

I think there might be a case that the mechanics of the game might have done some damage too.
They'd moved away from doing damage prevention and when they did away with protection, they took away a substantial tool in what a theoretical heiromancer can do.

Yeah, that bothers me. His ability to tap into universal order could have come very helpful when facing the Eldrazi.

Retcons and inconsistencies you say? NO.

He also was not invulnerable in the Purifying Fire. So.

He also didn't look Grecian/Western European either.