GPS not found is an error that occurs when your phone cannot connect to the internet, or the connection is so bad it cannot load GPS coordinates. This has happened to me several times while playing this game in different areas. If this happens, it could be caused by:
- Being out of range of/something blocking your provider, such as being underground.
- You've used up all the available data.
I'd recommend checking your internet. This is a common thing that many people encounter. And if this mainly happens around gyms, then there must be a bad connection around them.
It could also be caused your phone being damaged (which I highly doubt), or the servers acting up (also doubt).
Source: Personal experience.