Russell:

I live in Indiana in the US and we have red clay. The one plus for us
is that there is limestone in the area and that helps. Gypsum is
basically lime dust. If you spread it on the ground it will help to
break up the clay - over an extended period of time. The best thing you
can do is amend the soil by topping it with top soil or organic peat. I
got a new house last year that had virtually no topsoil as the lot was
scraped by the developers. Fortunately we got sod so we had some
topsoil there. However, I spread organic peat around some of the spots
that needed leveling and continue to do so throughout the rest of the
yard. This has had a tremendous impact on the health of the grass. This
spring, the areas that have the organic peat greened up weeks ahead of
the rest of the yard. You can also use peat moss but I found that the
price for the area covered is about the same and organic peat has alot
more nutrients. I can buy it in 40 pound bags for about $1 a bag. I
just throw them on the ground, cut them open, dump them out and then
use the back side of a garden rake to spread it out until it settles in
and I can't see it anymore. As far as aeration goes, if you are going
to be serious about it the best thing to do is rent a plug aerator or
have someone do it for you. If you do this, make sure you do the
aeration before you spread the peat. That way the peat can get down
into the holes and start breaking down the clay even deeper. It will
take a year or two to really get the full benefit of this but it is
definitely worth it. Hope that helps.

Jay