"Klara" wrote in message
(snip)
I also saw some things that
say that gypsum only works with certain compositions of clay soil (either
high calcium or low calcium or something like that but I can't remember
which, and on the wrong type it would make it worse!)"

Gypsum is calcium sulphate so adding it to high calcium soils just ads more
calcium. The purpose of adding lime/gypsum/wood ash is to cause the clay
particles to flocculate and make the clay "fluffier" or more open to water
and roots being able to get through the pugginess (not very technical but my
brain has gone to bed).

Add sand and sawdust and leaves and manure or anything that has has once
lived including old dead weeds, dead hens or old cotton Tshirts or green
manure crops. Tell her to not expect very much in the first year and even
if the veg she grows are pathetic, it will get better with each succeeding
year.