Forbidden from saying a thing.
Okay, this is going to be a long one, so sit tight. And yes, I can get boring at times in my explanations.
This theory will cover the wish, Marinette, and things that don't seem to fit the logic of the show. I will cover things as I go, so please don't reply until you've read the entirety of this post.
Marinette is forbidden from speaking the truth and she doesn't know it; to start this off, we will go back to the last episode of S5 "Re-creation" (my source for this Re-creation/Transcript | Miraculous Ladybug Wiki | Fandom)
Gabriel states: "Marinette, make sure that Adrien never knows about the villain that I was, (puts down the twin rings on the ground) but instead, that he remembers the times I tried to be a good father. (turns back to Gimmi)"
Alright now that we've got that out of the way, let's break down what Gabriel says here. He says to "make sure Adrien" quote on quote "never knows about the villain that I was" he said never, and what gives Gabriel enough reassurance to make sure she does so? he doesn't know her well. So, what I'm thinking is that part of his wish, was for Adrien to never know. Like said earlier, quote on quote.
Alright, you may say. "Well, you can only make one wish from Gimmi!" well here's something to also cover that.
Gimmi: What do you wish for?
Gabriel: Read my heart.
Gimmi: What do you sacrifice?
Gabriel: Read my soul.
Gimmi: So be it.
Alright, so, why is this important? well, Gabriel made a wish from his heart, what did he wish for? deep in his heart. He wanted Nathalie to be healthy again and to be reunited with Emilie. But this isn't the only thing. A true SACRIFICE from the soul would be something DEVASTATING. Like Adrien never knowing the truth... the show never explicitly states what Gabriel sacrificed other than himself, he wanted that-- but, yes, for sure that could've been the sacrifice. Himself, but he's getting to be with Emilie in return. The direct effect on the universe revolves around Adrien, it already has and always will, Adrien will never get to know because Marinette cannot speak it. She just doesn't know that-- you might as well also be saying. "WELL. That's just because of Nathalie saying they have to stick to the plan! Adrien can't know!" that's literally the universe going against her, that is one of the consequences of Gabriels wish.
I have something else I'd like to explain that will support this further. She was going to tell Adrien the truth one time but saw the frog when she came to Adrien in Sadnansi. You may say this was just a small detail that made Marinette change her mind, no, it's Gabriel. It's BEEN Gabriel. In another episode it also literally zoomed IN on the frog at the end... it's something left behind to make sure the truth never gets out. Same with Nathalie, these are all consequences and side effects on Marinette because of the wish.