I'll list a few good reasons to ship Gabrinette (Gabriel Agreste x Marinette Dupain-Cheng) from Miracolous Lady Bug.
1. The Enemy-to-Lovers trope:
The enemy-to-lovers trope is one of the most celebrated (and most beloved) tropes of all time, whether in books, fanfiction, anime, or TV series. It contains conflict, attraction, and drama. So everything you need for a good story. Hawk Moth and Ladybug are sworn enemies who have the potential to become lovers.
2. Time travel and different universes:
In "Miracolous Ladybug," time travel is possible through the Rabbit Miracolous. Marinette could travel to Gabriel's past. Or there's the possibility of an alternate world where they're both adults and married. Then the constant argument about the age difference no longer applies.
3. Gabriel appreciates Marinette's assertiveness:
Unlike most people, Marinette argues calmly and remains persistent. Especially because she lacks authority and status, this demonstrates her inner strength. Gabriel appreciates this.
4. Hawk Moth's obsession with winning against Lady Bug:
When he possessed the time-traveling Miracolous, Hawk Moth could have saved his wife from certain death. Instead, he focused on defeating Lady Bug. In that instance, Lady Bug was more important to him than his own wife.
5. Ladybug's emotional outburst:
Ladybug appears to cry when she ultimately fails to save Gabriel. The way their hands touch at the end creates a truly romantic moment. Furthermore, she is the only person who has ever managed to change his mind.
6. Betterfly:
Betterfly (Gabriel's good version) also seems to respect Marinette and treat her as an equal. And the scene in which he rescues her can also be interpreted in a romantic context.
7. Her Potiential:
Perhaps not so important, but still worth mentioning... Gabriel appreciates Marinette's fashion creations. Which is high praise indeed, coming from such a renowned fashion designer as himself.
Feel free to add to this list.