When I used to do the home wine making thing. I read that it was not possible to get the alcohol level above
21% by mere fermentation as at that level it kills the yeast, thereby halting the process.
Which is why strong Port and Sherry are called fortified wines (They are fortified by adding Brandy or other spirits after normal fermentation has ceased)
Liquor at levels higher than 21% are produced by distilling so I am not sure that you can increase the percentage alcohol level of gin which is a distileld spirit.
The Sloe gin I have made has certainly increased in volume by the amount of dissolved sugar added but I don't think it became stronger, the problem is it tastes like pop, and is very potent because you can drink pop in large quantities. Hic!