If you only want the group name shaded, put it in a text box and format that rather than shade the section of Group Heading.
You can make it grow but this isn't ideal if you don't want wrapped text, but want all headings on one line and only shaded to the end of the text.
You could conditionally suppress several text boxes of differing length (and/or colours) , laying them over each other in the same place
so for Box1: choose format, suppress formula and enter
length(groupname)<>5
so that text box (& by default the colour) will only show when the group name is 5 characters long
I have a similar rule I use so company logos show against the correct supplier:
I suppress each image , place them over one another in the group header
with each having the image formatted to suppress with a formula
{Table.Field)<>"supplierA"
so the image only displays if group's supplier = SupplierA