I have a problem with a garden bed that gets really soggy in heavy rain.

I had assumed the soil had too much clay in it (we've a fair bit of clay
in the area) and had planned to use some gypsum to fix this. However,
yesterday I had a good hard look at it (and did some digging).

There's about 6-10 inches of nice, slightly sandy and obviously imported
topsoil on top of a sheer sheet of clay. Hence my problem with soggyness
after heavy rain! It just doesn't drain anywhere!

My question: would gypsum have any impact on this clay, 8 inches down under
the soil?

Second question: are there any plants gypsum will hurt? Concerned mostly
about roses, lavender, and ajuga.

adt.