I can add to this.
Generic probes are added to all beginner kits only to keep the initial buy in price of the GHL packs down as best as possible. However this does not mean they are poor quality. The probes we use are tried and tested by me personally before we adopt them for GHL, this is important as GHL is a very high brand of controller and the last thing we want is a useless probe going out with the controller.
However they do have a shorter life than a GHL and will not respond as fast as a genuine lab grade certified pH probe such as the genuine GHL pH probe.
The generic probes are basically supplied to dealers at OUR cost (Free) so if a client wishes to upgrade the pack to a GHL pH probe they can in the knowledge they are not losing money, in fact we ask all dealers to still send the generic probe if you do the upgrade so you end up with two probes.
Generic probes are not sold seperate but are also included in pH expansion kits when purchased.
I have tested more probes than I have eaten bananas and I have NOT yet found another generic probe that comes even close to the one we use.
So you have two options, stick with the generic until it starts to react slower (about 6 months) or upgrade at time of purchase.
The generics do not have a warranty however if it failed in the first month we would honour it. where as the GHL pH probes come with a 3 month from time of purchase warranty.
My advise is go for the GHL pH probe from the start if funds allow, you then have a probe quality that matches the controller.
In my tank build I am using the generic probe and have been for 7 months, I have calibrated it twice in this time