You can use either fixed function UART or UDB based UART for this application.
Which PSoC family are you using?Some of the PSoC families wont have UDBs. In such cases you need to use fixed function UART only.
Do you need two way communication between PC-PSoC and PSoC-gsm modem interfaces? In those cases, you need to use two UARTs in your applcation.
Please refer UART_TX, UART_RX code examples in PSoC Creator for basic understanding.
You can also refer CE95366 from the link below:
Please try and update if you have any queries.