If you go to the GPG Options page (described above) and enter the mount period in "Major Period", and click apply, then KStars should remember it.
If "Estimate Period" is not checked, then the period should not be updated by GPG.
As I said, either look up the period if it's on the PHD2 Wiki page above, or keep a good estimate from a successful KStars GPG session that lasted
a long time, e.g. over an hour since the last slew or other GPG reset.