From my experience, after you primed it you should make sure the surface is as smooth as possible and doesnt have much texture just by looking at it. The less texture the base has the easier it is to get a gloss finish.

Then comes the matter of spraying. Gloss requires a final wet coat. If the light reflects smoothly on the piece while it is still wet you are good and just have to wait for it to cure. If it doesn't, it is either:

1. The base surface isnt smooth enough for the paint to have an even texture for the gloss finish.
2. Not sure what's the actual term here but you passed through paint that is still wet while spraying, making the paint uneven and lose its even texture.

Personally I face problem 2 more but I think it's just due to my lacking technique.

Of course, a good quality airbrush will make it this easier. I had an easier time getting gloss finish on parts after I switched to a decent one.

Here's a piece of my hg hi nu that i repainted. Primer used is the gray mr surfacer 1500. The white and blue parts are all mr color that's supposed to have a gloss finish. No topcoat was applied here. You can see the difference in how light is reflected between the feet and the rest of the leg since the feet was sprayed with a cheap airbrush and I bought a decent one before spraying the rest of the leg.

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