If the Guard wasn't used, why is there a monument to them at Omaha Beach?
Exactly.
Below is a list of National Guard divisions, the theater in which they served, their activation, sent overseas, enter combat, and inactivation date(s):
26th Infantry - Eur - Jan 41, Sep 44, Oct 44, Dec 45
27th Infantry - Pac - Oct 40, Mar 42, Nov 43, Dec 45
28th Infantry - Eur - Feb 41, Oct 43, Jul 44, Dec 45
29th Infantry - Eur - Feb 41, Oct 42, Jun 44, Jan 46
30th Infantry - Eur - Sep 40, Feb 44, Jun 44, Nov 45
31st Infantry - Pac - Nov 40, Apr 44, Jul 44, Dec 45
32nd Infantry - Pac - Oct 40, May 42, Sep 42, Feb 46
33rd Infantry - Pac - Mar 41, Jul 43, Sep 44, Feb 46
34th Infantry - Med - Feb 41, Jan 42, Nov 42, Nov 45
35th Infantry - Eur - Dec 40, May 44, Jul 44, Dec 45
36th Infantry - Med & Eur - Nov 40, Apr 43, Sep 43, Dec 45
37th Infantry - Pac - Oct 40, Jun 42, Jul 43, Dec 45
38th Infantry - Pac - Jan 41, Jan 44, Jul 44, Nov 45
40th Infantry - Pac - Mar 41, Sep 42, Apr 44, Apr 46
41st Infantry - Pac - Sep 40, May 42, Jan 43, Dec 45
42nd Infantry - Eur - Jul 43, Dec 44, Dec 44, Jul 46
44th Infantry - Eur - Sep 40, Sep 44, Oct 44, Nov 45
45th Infantry - Med & Eur - Sep 40, Jan 43, Jul 43, Dec 45
Most of these units served in WW1, at least two served in Korea and/or Vietnam also. The 29th took part in the D-day landings in Normandy.