To reach Gatwick on a budget, take Thameslink from St Pancras, Farringdon, or London Bridge

https://edwinhxtn094.cavandoragh.org/gatwick-airport-by-car-best-routes-traffic-tips-and-costs-1

To reach Gatwick on a budget, take Thameslink from St Pancras, Farringdon, or London Bridge. Trains are frequent, accept contactless and Oyster, and often cost less than express services.