i have cloned the repo https://github.com/PrefectHQ/server and installed the dependencies and than ran the command prefect server start and prefect server is running on localhost port 8080. Now how can i register flow?

In the same way as you did when you were using Prefect Cloud. However, you need to adjust config.toml on the Server this way:

[server]
  [server.ui]
    apollo_url = "<http://YOUR_MACHINES_PUBLIC_IP:4200/graphql>"

and on the machine from which you register:

[server]
endpoint = "YOUR_MACHINES_PUBLIC_IP:4200/graphql"

You need to do

prefect backend server

also

yeah it is working locally. now i am going to deploy it on GKE