P2Pool does not have a JSON config for normal mining. The old config.json is gone. Do not edit sidechain_config.json. That starts a private chain and you mine alone.

Live options are command line, or a params file that is key = value, not JSON:
https://github.com/SChernykh/p2pool/blob/master/docs/COMMAND_LINE.MD

The JSON you want is xmrig's. That is the live screen.
https://xmrig.com/docs/miner/config
https://xmrig.com/wizard

You do not even need the file. Stop XMRig inside Gupax. Leave P2Pool running. Then in cool-retro-term:

./xmrig -o 127.0.0.1:3333

If Gupax is not on 3333, use the stratum port it is already listening on.

Wallet in xmrig is ignored. Gupax already set it on P2Pool. Leave it there.

If you want it saved, the pool block is:
url 127.0.0.1:3333
user x
pass x
keepalive true
tls false

Do not start that while Gupax's miner is still up. Two miners fight for the same cores. Tail was the log. This is the flow.