During install fromm 22.04 media selected button (Do something different)

[hput]

Installing from 22.04 media ... after selecting the zfs option.
There is a choice to proceed or a second
  choice {X} "Do something else"

It also says changing parition sizes and more I don't have at hand.

Choosing that button... I'm trying to resize the main partition of the
the 4 the installer makes.

Using Gparted (as root).  Attempting to resize the main partition of
some 220 GB.

I want to shrink the main partition but the option to resize although
available to choose, once choosen in Gparted... the option to resize
id greyed out

I don't understand why gparted would have important choices greyed
out.

It seems first the installer offers to give me a chance to change the
partitions ... and then makes it impossible.  Or at least Gparted will
not allow any changes in size.

I'm quite sure it is my own doing somehow but not sure how.

Is there any way to modify the partitions the installer creates?