So you've just done a DCCD transmission swap in your 2015+ WRX, and the gear indicator on the gauge cluster is not working properly.
Not to worry, this is perfectly normal. You may see something like the situation below for the gear display.
Gears 1 - 4 show fine
Getting into 5 shows gear 4
6th gear is fine
The gear indicator on the cluster is based on the ECU's calculation of vehicle speed compared to RPM. With the new transmission, those ratios are not correct anymore. Therefore, the ECU is no longer calibrated correctly, and the gear display doesn't work properly as a byproduct.
The solution is simple! Reach out to your local tuner, who can adjust this in the ECU software to match which gear display to the new ratios to display the output correctly.