Will a polyclonal antibody attach to proteins of different kDA? If I have a GST 26kDa polyclonal antibody, will it bind to the GST 28kDa protein as well?

If the sequence of the 26 kDa form is included within the sequence of the 28 kDa form then the answer is 'probably yes'.

However if the N- or C-terminus of the short form is an important/dominant epitope and it is masked in the long form (by being extended) then the activity of the serum against the long form could be significantly reduced.