I have more than a passing familiarity with the interviews for Y1 at GSIS. Not sure what specific advice you would be looking for, but in general I would certainly be recommending to anyone that first and foremost they be choosing the right school for their child -- sometimes we can get caught up in trying so hard to help them pass that we forget that the school has to "pass" too. For example, in the case of GSIS, make sure the eventual combination of mandatory German and Mandarin is right for your family.
As you may know GSIS may be moving to Ma On Shan on a temporary basis. This move may suit your location or not...
The interview itself, which I am sure the school's admission office will freely tell you, entails taking a group of about 8 students to sit down with a group of teachers for an hour or so. As a parent you will wait and be given a 'talk' on the school by the Admissions staff and perhaps a member of school management.
During this time the children will be observed playing and interacting with other children while they play with some provided toys, and will be taken in turn to do some activities with the teachers: identifying letter names, sounds and possibly some words that begin with that sound; number concepts including counting forwards and backwards, before and after, etc; and some discussion intended to determine the child's level of English, which the school requires to be "equivalent to that of a native speaker".
Most of the academic content of the interview is certainly taught by Kindergarten programmes throughout Hong Kong, so if your child has picked up on this they should do well on these aspects.
Why children do not pass: My understanding (such that it is) is that children may not pass due to 1) non-native speaker equivalency, 2) readiness issues -- more common in May, June, July and August birthdays -- the youngest in GSIS's cut-off dates, 3) lack of age-appropriate academic knowledge 4) Potential presence of a special need that the school cannot service.
Some of this is just surmising but I hope it is helpful... I think most children just view such school interviews as a chance to visit a new school and to talk about themselves. My guess is that "over-preparation" or training will come across to interviewers as just that, and will likely make a child more nervous than prepared, and an interviewing teacher feel awkward, and ultimately stand in their way of getting to know the "real" child...