You will get lots of debate in this as it is very much a preference thing. Typical ports are fortified with high proof brandy not the same 40% brandy you buy to drink. A high proof spirit will dilute the wine less. Brandy also has flavord which match well with ports but I'm not sure it would match well with gooseberry especially if you have to add a lot to bring your alcohol level up.
This is my opinion. If it was my wine I would use something high proof and with neutral flavors so the gooseberry flavors can shine. Around here I can buy 95% everclear which is almost pure alcohol. That's what I would use. Minimal dilution and less alteration of the gooseberry flavor.
For a fruit wine, just use Everclear (95% ABV). I did, it works fine. For those who are not licensed to run a still, it is hard to do it any other way.
If you want to add even more gooseberry flavor, make a gooseberry liqueur but use pure grain instead of vodka. It is a sweet liqueur, but you could skip the sugar if you wanted to. I made this a few years ago & it was fabulous. Sadly, my source killed their shrubs, so I have decided to plant some. http://ewainthegarden.blogspot.com/2009/08/homemade-gooseberry-liqueur-for-mental.html?m=1