Deck grip

Mozzy,
Sand back just enough to get it flat and “keyed up” use a da sander if poss. 120g.
Clean / acetone.
Yes mix the griptex with the catalysed gel. Griptex are plastic granules like fine sand.
I’d use medium (I have some!) if it was a sitting area I’d use fine Esp on gel which is thick.
Use a generous amount of gelcoat on roller don’t be shy, I think it probably doesn’t need thickening that’s all.
Very much Worth practicing on some scrap to get the effect you’re after. How heavily loaded roller is, thickning of gel, proportion of griptex.

Marineware.
A half litre sized tin is about £50. It’s ridiculous.
For info I did the entire i14 non skid area (dancefloor, foredeck, gunwales) with about a quarter of said Tin. So you don’t need loads.