Fill with color in Geometry App

[parisse]

Because L1 is a list, you must convert it to a sequence in order to flatten all the arguments of polygon. Try
GC:=polygon(op(L1),('display')=(3+filled))
Or use a CAS sequence to store your vertices:
l1:=0,1+i,2*i,-1+i/2;
polygon(l1,('display')=(3+filled))
Note that when using CAS variables, the input is kept exact, e.g. perimeter(l1) will return an exact value.