By default, Guidepoint Global appears to utilise a per-minute based billing, but does allow the consultant to round up consultations on a 5-minute basis. There are no minimums on the platform.
However, as a special case, they do let you set up your profile for a 1-hour-based billing, although that does appear to require a manual pre-approval, and might also limit your consulting opportunities.
However, as far as the rates are concerned, it appears that it's mostly used by the medical professionals (partly owing to their origin as pharma-investment consulting), for whom 15-minute-based interruptions are more of a norm, and thus, it appears that as a Software Engineer consultant, getting our industry's standard of 4× the regular hourly rate, together with the hourly-based billing, is not an accepted a practice at Guidepoint.
It appears that most sources suggest that the hourly rates the consultants get from the platform is 200—400 USD / hour (billed in 5-minute increments, with no minimum, and most consultations being under an hour), which does seem to be on the lower end of the Software Engineer consultants, which ordinarily charge 100—200 USD/hour with a 4-hour minimum.