Coming from a road bike design the majority of gravel bikes take a 27.2mm seat post. This immediately limits your choice of posts with the majority being 30.9mm or 31.6mm, the larger diameters better accommodating the dropper mechanism and are the standard specification on MTBs. Remember you can always shim up a narrower post to fit a bigger frame but not the other way round.  Shimming is a useful option to get a shorter travel post in a bigger seat tube frame as it’s unusual to find a post less than 100mm travel in 30.9 or 31.6mm diameter (because there isn’t the demand for short travel posts on ‘bigger’ bikes). Conversely you’re unlikely to find a post of much more than 100-125mm drop in 27.2mm as it’s not possible to fit the bigger workings into the smaller diameter.