Radak (1): Then he was alone (Malbim - and Achish thought that he is a spy, and there is no benefit from one man). Now he had 600 men with him, and before going to Gas he sent to Achish and said that he is repulsed with Sha'ul and his nation; if Achish wants, David and his men will stay with him and help him fight against Yisrael; Achish consented. Also refer to 27:3:1:1.