Coinbase maturityAccording to the block chain protocol, generated coins can be spent only after 100 confirmations.
Users should be aware that fetching a wallet's balance from the client returns also immature coinbase transactions.
If you provide such coins to the gocoin wallet, the network will not mine your signed transaction until the coinbase inputs that you included in get 100 confirmations.
Mining APIThe mining RPC API is experimental and had been implemented in March 2016 - far before the segwit support was added.
It was tested on Testnet3 and worked well, but only for one mining setup. It has not been used ever since then.
If you are interested in using Gocoin node for mining, please contact me (email@example.com) so I can help you with any issues that may need resolving these days.