How to use a node on one server, and a GPU on another? Is there an RPC port for connecting between different servers?
There is HTTP API that you can use to query for work to mine on.
There is documentation: https://github.com/aeternity/protocol/blob/master/epoch/api/mining_api_usage.md
And here is API spec: http://aeternity.com/epoch-api-docs/#/external/GetPendingKeyBlock
In the future it will be upgraded to some standarized way of pooling like stratum.
Of course you can hack the node and use lower level RPCs to query for work from worker server.