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.
I have a question about the GPG period in seconds. I noticed that it must be an integer. If the period of my mount is 239.5 seconds, then after an hour (15 periods of 240 seconds) it will be 15*0.5=7.5 seconds off a multiple of 240. Does that mean that my GPG will then be out of sync by 7.5 seconds if I enter 240 as the period, and don't click the "estimate period" checkbox (as recommended)? If it does, can the integer period be changed to a floating-point number?
One last question: How is a linear trend accounted for? IMHO it should be considered separately as there will always be some trend due to polar misalignment, atmospheric refraction, clutch slippage, etcetera. A trend is a non-periodic signal so when not properly accounted for, it may to additional errors rather than reduced errors.
In particular, he does mention how there's both periodic error correction as well as correction for drifts built into GPG. Also, of course, there is standard control built in to the guider to account for other types of "random error".