I don't think the WHO guidelines reference gut development at all when it comes to solids at a 'normal' age i.e. after four mths. I have not checked today, though.
It's known from previous research that very early intro of solids ( before three months) may have risks, but the research on later solids (after four months) is just not nuanced enough to show a difference between four months and six months. The research that WHO used was the Kramer and Kramer work which showed there is no harm to babies who are breastfed in having solids 'delayed' to six months. There is nothing much at all about formul or mixed fed babies - policies are based on inference only.
Six months is a sensible guideline for most BF babies - very few show any signs of actually needing solids before then, so with BF babies, why give them something that could be inferior in quality and nutrition before then? OTOH, babies under six mths take only tiny quantities so we prob don't need to worry, esp if solids are not being shovelled in :) and parents are careful to go at the baby's pace (one of the big plus points of BLW, really, is that the baby is in charge, so no risk of over feeding).
Ff or mixed fed babies - some authorities think they might need solids earlier than BF babies, because formula does not change with the baby's development the way breastmilk does, and does not acquire subtle flavours that act as an intro to solid foods.
There is also recent research that shows we probably don't need to be so concerned about allergy or intolerance (one of the worries has been that early solids might pre dispose to this, but this does not appear to be the case as long as solids are not intro'd very early).
The basics are, I think, that for most babies, watching the baby is better and safer and happier than watching the calendar. For most babies, they will show signs of active interest and curiosity and ability to self feed round about six mths.