Well, yes, but no.
This used to be an option in the old GRP rule 4.1, where it stated that
on-line ATC shall provide a “top-down” service and whenever traffic load permits cover for any missing control positions beneath them
but that’s not the case anymore. The text in bold has been removed in GCAP 4.6, thereby effectively enforcing that a controller shall provide top-down at all times, regardless of traffic load.
I remember this caused quite a bit of discussion on Discord from the moment the first GCAP drafts were publicly released, but it was never adjusted to allow traffic load exceptions for regular controller positions. The only allowable exception (to nobody’s surprise, unfortunately, but that’s another discussion) is under rule 5.6(d), i.e. when you’re staffing a so-called Super Center.
Which is probably because what was made publicly available was a final draft without any intention for further changes to be made based on feedback by the broader community. However, 4.6(c) does provide a way to not provide topdown service: vACCs can define positions that don’t provide topdown services, but these positions can then only be staffed if someone is online below who does provide topdown service (kind of similar to what we currently do here in Germany and presumably you guys in Belgium and the guys in the Netherlands as well with EDUU/EDYY sectors where they are usually covered bottom up by EBBU/EDGG/EDMM/EDWW/EHAA but only really separately staffed once one of these stations is already online) or if you have special approval from the vACC to open it without a station below being online. And surprisingly, I can’t find anything in GCAP that would describe how a vACC is held accountable in this regard or that precisely defines when an event would be facilitated by this or how we are to interpret “time-limited traffic situation”, so vACCs could theoretically define positions that don’t provide topdown and whenever there’s a lot of traffic, controllers could open them without having to provide topdown.
But since you’re mentioning GCAP, there are a few points, that could have been improved with such a new policy (or potentially can still be improved in potential future updates to it) in regard to the original topic of this thread, namely the endorsement policy and controller competencies (particularly pertaining to who is allowed to staff what types of positions). If both were more flexible, this could greatly aid in alleviating workload during events and making them more enjoyable for both controllers and pilots.
On the one hand, before GCAP, there were a number of vACCs who used separate event endorsement so trainees were only allowed to participate in events without a mentor once they had proven that they can deal with the added workload and complexity whereas GCAP now expressly prohibits such an endorsement which means that especially during regular (weekly, biweekly, …) events at unrestricted airports it will become more likely that controllers are not able to deal with the workload/complexity, which will obviously negatively impact the experience of both pilots and adjacent controllers. On the other hand, particularly here in Germany we see how well controllers do even on stations with responsibilities that are technically above the required competencies for their current rating as our S1 minor program allows S1 trainees to staff most smaller airfields up to TWR without a solo (which not only makes it more obvious to pilots who to call at these smaller fields - before this program, those fields were essentially never separately staffed - but also reduces the workload of the radar controllers as they don’t have to pay attention to these smaller airfields and can concentrate better on their primary tasks) and the tightly packed airspace results in APP sectors that - due to the sector structure - have to presequence traffic inbound for a neighboring TMA (which always works without bigger issues, even during events at these neighboring airfields, and suggests that S3s may be able to provide adequate service on ACC sectors that presequence a specific traffic stream for a given airport, which again would reduce the workload for radar controllers, particularly those precious, rare C1s). If GCAP had not gone in the restrictive, overreaching direction it has but rather left more flexibility for vACCs to adjust their staffing rules and training to local needs based on their expert knowledge of the local airspace, procedures, and difficulties, they could have taken steps to simultaneously increase coverage for all pilots and reduce workload particularly (though not exclusively) their C1-rated controllers.
