As already mentioned, there is the translation of the Gesta by Elisabeth van Houts. But as far as secondary sources go, anything by van Houts is good. She has a forensic style and possess a knack of ferreting out sources, and of getting as much information as possible out of them. She wrote an article on the work of William of Jumieges in Anglo-Norman Studies 3 (1981), 'The Gesta Normannorum Ducum: a History without an End'. She also wrote an article in French specifically on pre-1066 Anglo-Norman relations for Les mutations socio-culturelles au tournant des Xie-XIIe siecles, ed. R Foreville et al - although this may be somewhat difficult to get hold of.