It’s better to think in positive terms in GAMS, in my experience.
ie
I want to remove elements in set B from set A
==> I want to create a set with elements in A not in B
Which is done in this way:
set A /a1*a10/;
set B(A) /a1*a3/;
set C(A);
C(A)=YES;
C(B)=NO;
gl