For some reason te test case pases with floats. I changed it as suggested to:
assert GramSchmidt(L) == [Matrix([
                                [2],
                                [3],
                                [5]]), Matrix([
                                [ S(23)/19],
                                [ S(63)/19],
                                [ S(-47)/19]]), Matrix([
                                [ S(1692)/353],
                                [ S(-1551)/706],
                                [ S(-423)/706]])]