The problem of the right generalisation of Gray-tensor product is notoriously hard. In homotopy theory an analogous status is achieved by the problem of definition of a tensor product of A_{∞}-categories (or A_{∞}-algebras). Tensor product of E_n-algebras is even more mysterious.
In my (not always formal) talk I'll try to suggest a general method for constructing such tensor products based on our joint work with Cisinski and Weber on multitensors.