Hi Mateusz,
The RSSI value could be read it, during scanning process using gapScannedDevice_t, but You will have to check all the adv package and look the only that you want.
During a connection you can look for the RSSI value;
typedef enum gapRadioPowerLevelReadType_tag
{
..
..
..
gRssi_c /*!< Reading the Received Signal Strength Indication in a connection. */
} gapRadioPowerLevelReadType_t;
Best Regards,
Mario