The usual suspect when circles are not circles are the v-wheels. Check all of your v-wheels and adjust and/or replace as necessary.
If the v-wheels are good then a calibration may be necessary. The gbrl sets the stepper motor pulses to move a given distance. To calibrate make a series of circles that within your caliper measurement range and cut them. Then measure them and if necessary adjust your stepper motor numbers to correct to make perfect circles. The calibration is rarely needed but is available if required.
If you make a series of circles and the measurements are consistent then you may need to calibrate. If the series of circles are inconsistent in measurements then you have a mechanical problem and not a calibration issue.
Take a look at this community post about calibration.