How much time should one allow to change trains from the Eurostar platform (from Paris) to platform A at St Pancras going to Gatwick?
In general I'd budget at least 30 minutes for that connection, but it'll depend on wether you're a fast walker, if you have (heavy) luggage, if you know the station already and if you need to buy tickets for the Gatwick train.
The Eurostar arrival takes you down an escalator and through a maze of corridors with immigration and customs facilities (which aren't in use, as you clear UK immigration in Paris already). This is different from Paris, where you just walk from the platform into the station.
You'll also have to find your way to Platform A once you're in the station - if you accidentally end up at the wrong side of St. Pancras this may be a bit of a walk as well.
For tickets you should be able to use a contactless credit card (or Oyster card) at the ticket barriers. This is probably the quickest and most convenient option by far, as you can just "walk in", although it'll always be the standard fare without discounts.
Buying tickets locally may take some time. The whole fare system from and to Gatwick is a mess and you may need some human assistance, either a manned counter, or from a guide at the ticket machines. (I'm not sure about getting tickets in Paris (or on the Eurostar)).
That said, services to Gatwick should be quite frequent, so you can just take the next available train if your faster or slower than expected.
Finally, you should also take into account possible delays - which are not too uncommon - on the Gatwick connection and the time it'll take to get to your gate in Gatwick.
 At the time of writing, Thameslink quoted the "Anytime" flexible single ticket at £10.70, while a "Offpeak" saver single fare was at £9.40
The minimum connection time at St. Pancras according to the timetable is 15 minutes - from the Eurostar Arrivals exit, turn right and you'll see the Thameslink platforms signposted (note you'll want to go downstairs to platforms A and B for Gatwick Airport, the trains upstairs head north).
If you also need to buy tickets it would be wise to leave at least another ten minutes to do so, but you should be on the Thameslink platform within 25 minutes of arrival without too many issues.