ALL trains in UK are valid with your pass with the exception of Heathrow Express and local metro/underground/tram networks.
You can take any train from Gatwick you please without reservations.
Trains Direct to St Pancras run several times an hour under the Thameslink brand. They all run through to other destinations, the best ones to get are the fast trains with 5-6 stops between Gatwick and St Pancras, These are mostly trains with Bedford or Cambridge as destinations. There are also slower trains with more stops, these usually get overtaken by the following fast, there is no problem taking these but they take around 60 minutes with the fast taking 45.
All that is based on the regular weekday service pattern, as you haven’t said what date there may be a different or even altered service running.
Gatwick station has plenty of staff, if in doubt just ask for assistance.
The app planner is not always accurate, it is always better to use local resources for planning.
https://www.nationalrail.co.uk gives times and updates for all British rail services.
There are wide scale strikes planned for 15, 17, 26 and 27 September in UK, travel anywhere on these days may be disrupted or impossible.