How do i change my pokéstop to a gym
Pokestop have chance to turn into gym when the number of wayspot within certain cell reach certain value.
2 wayspots >> 1 gym
6 wayspots >>> 2 gyms
20 wayspots >> 3 gyms
You might want to read here
https://pokemongohub.net/post/article/comprehensive-guide-s2-cells-pokemon-go/
Please know that sponsored gym count towards these number