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

https://miloanod546.wpsuo.com/gatwick-airport-railway-station-everything-you-need-to-know

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.