On the GRT Setup display, check "Serial Port 5 Input Counter" to see if it's counting up.
If not,
1. confirm that the baud rates actually do match
2. GRT serial port 5 is configured for ADS-B
If it's still not counting up, you may have a poor/wrong connection.
Also, on my GRT/ECHO system, the ECHO output "default protocol" is set to "GDL90".