Prior to the Siege of Petersburg, Gilbreath was given command of 800 engineers to create the breastworks for the siege. This was done during the night of June 21–22, 1864. The Regiment took part in the siege until the end of July, when they were pulled back behind the lines for rest. In August it was decided to make a feint towards Washington. The regiment marched toward Washington, and was loaded onto ships and taken closer to Confederate lines. The following First Battle of Deep Bottom saw the 20th Indiana capture a hidden Confederate artillery battery. This was done by Capt. Gilbreath, who tied a red handkerchief to a stick, mimicking a signal flag. The Confederate batteries mistook Gilbreath's handkerchief as an actual signal flag, opening fire and giving away their position.