Welcome in the forum, Patrick!
Regarding the communication between the PSoC BLE and the WonderGecko:
I2C has a bus master, so you will have to poll at the master side (PSoC) if there is a message ready from slave. Polling costs energy.
UART can be set up bi-directional and you may disable the component and Hi-Z the pins at need to save energy.
For the over-the-air update there are example projects available, just search at top of this page, but be warned: It might take some precautions (EEProm/Ram) to make it foolproof not leaving the customer with a non-functioning device caused by a broken connection.