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

https://telegra.ph/Step-by-Step-Getting-from-Victoria-to-Gatwick-Airport-by-Train-08-21-3

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.