Gerard was the colonel of a Royalist regiment of foot (his twin brother, Ratcliffel,was his lieutenant-colonel) that was already in the field before the first major pitched battle took place.
After the advance on London and the Battle of Brentford, Gerard marched with his regiment and that of Molyneux to Brill to take up his appointment of Governor of Brill and to oversee its fortification and to defend the place against incursions by Parliamentary forces.