It sounds like you aren't actually forming your Gilman reagent. Have you tried to form your alkyl lithium just from the alkyl halide and lithium metal?
Also a quick search of Sci-finder has very few examples but they usually form a Grignard first.
Going off this paper: Huber et al. J. Organomet. Chem. 2013, 744, 144-148. see pic related.