OK, so you haven't tried it yet? Given your previous comment asking about hand-brushing it, followed by the photo, I thought you had already tried it.
If you haven't yet, and you definitely want to hand-brush it, I would absolutely thin it. Generally I thin anything I want to brush by hand, just as I would for airbrushing. That's something I learned over time, painting toy soldiers, and when I started using Tamiya acrylics. Picking up the paint right from the jar with a brush, I had most of the same issues you read people talking about. Once I figured out to thin them, and thin them with Tamiya's proprietary thinner, I got results as if I airbrushed them.
And then I realized that generally, when handbrushing, especially large areas, it helps to thin the paints and apply thin coats, building them up as necessary.
The only product I don't thin as a matter of course is Future. It's self-leveling, so it brushes pretty well.
For this gloss medium, I would thin it. But I would also test it out first. Get a piece of another kit, a piece of styrene sheet or card, lay down your color coats just as on your model-primer, color-and then test out brushing the gloss medium.
I don't know of anyone who has tried applying this by hand, but it can't hurt to test it out.
