The major difference is

(1) Basis sets, Some codes use slater type orbitals, some use Gassian basis and some plane wave basis.

(2) The full potential and the psoudo potentials are all basic difference for codes.

(3) Different codes use different integration techniques

Best wishes

A couple of other major differences are

Gaussian is a finite system code (molecules)

QE, siesta ans vasp are periodic systems codes

In addition Gaussian as many features not avalaible in the other codes. It is however pretty inefficient numerically.

Thanx Sir