Just what I was about to suggest Phonsman, though I am not familiar with Scribus.
What would be really nice (my way of saying essential) was if columns & rows setup was in the new document dialogue box. Most of my work is print based and I need a grid (not that graph paper thing, a proper grid) for laying out the pages... from the start. One of the other posts seemed to suggest making a multiude of templates with variations of guides and margins for a range of page sizes; this is not an acceptable answer.
The lack of setting a grid from the beginning is seriously going to impede my workflow, and would in fact, be a deal-breaker. I have been looking forward to Publisher's arrival, want to quit InDesign, but...
One last thing, if you (Affinity) do adopt Phonsman's suggestion, please use the word 'gutter' not 'gap'.