Please checkout the Betaflight github documentation about GPS rescue before using it to fully understand it's functionality:
https://github.com/betaflight/betaflight/wiki/GPS-rescue-mode
Joshua Bardwell also released a video with loads of information about this feature:
https://www.youtube.com/watch?v=B_Yl0wqRf68
Below our flyers that come with every GPS pre-installed BNF to make your FPV journey as easy and safe as possible!