Modern Vespa

Will this be good for a 2024 GTS 300 gear box? I've read it's gl-5 rated

Why not use something that matches the user manual?

Gear oil 80W-90
SAE 80W-90 API GL-4

The final drive is one of those places where it really doesn't matter what you put in, as long as there's something there. You could put in motor oil, or any gear oil that doesn't have a label that reads "dissolves steel and eats holes in aluminum," and be fine.
If that's what you have, use it. NHNF.

For gear oil in my GTS I have always used a Castrol 75W90 full synthetic that is both GL-4 & GL-5 spec.

The GL-4 required spec is probably more important than the 75W vs 80W difference.

https://rymax-lubricants.com/updates/the-differences-between-gl-4-and-gl-5#:~:text=GL%2D5%20has%20roughly%20twice,aggressively%20towards%20bronze%20and%20copper.

In my other Piaggio based scooter I use Motul HD 80W90 GL-4/5.
Either are fine as they meet the GL-4 spec.

Ive been using Redline MT90 75-90

When my 250 engine died at 74k miles I took it all apart and the drive gears were in perfect condition.