The code of contract is in the ckb-gpc-contract/main.c.
You needs to have two ckb account that have sufficient ckb in you local chain, and the mongodb.
cd client/test
ruby send_contract.rb
Note that send_contract.rb will print two info, the first is the code hash, and the second is tx_hash. You should remember these info, and change the @gpc_tx and @gpc_code_hash to your local version.
See the command in GPC by
./GPC
Please init in the first use
GPC init <private-key>
Every client should run
GPC monitor <public key>
GPC listen <pubkey> <port>
Then you can make payments by other command.
Note that some commands require continuous user interaction, please enter the corresponding content in your command line.