One foot by 8 inches is a lot of area to be rough sanding. Plus, I say again that you likely need to spray after the rough sanding to blend the gel in. This is not complicated, but it does require some technique.
If for some reason you actually can wet sand from 320 down to 1000 grit and not bleed through at some point in either the new gel or old, then you can go polish out with compound and then actual polish. I like 3m supercut, then finesse. Once you get through the supercut you can see whether you did a good job or you can see artifacts or not. If you do, break out the 320 and repeat. If you bleed through, concentrate on getting things nice and flat and fair. Resist the urge to stop using a block and create wavy spots with your fingers. Once flat, and well sanded an enlarged area into good gel coat, get a sprayer and spray a very nice smooth coat onto the flat surface. One you do that, you should be able to just knock the orange peel off the top with 400-600 grit, then work through the grits up to at least 1000 then cut/polish. I don't work anything more than 800 until I've polished at least once to see any "artifacts" that will require more spraying, cause why do the work when I'm going to need to do it again anyway.
To say it another way. In a decent gel coat job of something like 10 steps, we just described steps four through ten. Your gel guy did step one through three, which consists of rough hogging out the old surface and filling with a rough filling of thickened gel. Or again, how do you say in your language...."he did the easy part". The parts that require some skill, experience and feel were not done. You don't hire a gel person for steps 1-3. Anybody could do that. Heck, your not even at the point where you even know whether the color match is right as you don't really see that until the polishing is done.
If this gel guy is too expensive or you can't get him within two weeks (lord knows my gel people would not do a rough application requiring 60 grit to sand)then find another gel guy. If you do it yourself, you really need some help in these stages to get it right. I might do the above steps 2 or 3 times to get it really right. That's because I've had a lot of help from a really good gel person who corrected my work and gave me lots and lots of pointers. I'm a lot better than you. You might not get it in 2 or 3 tries. Granted my gel color is tough to match. We start with matched gel and then final adjust it on site for each area of the boat.
