Budget travelers can use Megabus or other low-cost coaches to Gatwick at select times

https://www.scribd.com/document/1077066259/Heathrow-to-Gatwick-Airport-Transfers-Coach-Taxi-or-Train-174457

Budget travelers can use Megabus or other low-cost coaches to Gatwick at select times. Fares vary widely; flexible dates and early booking usually score the best prices.