There are good GI tables out there. If I remember right, channa dhal is much lower than lentils and the lowest-GI legume of all is one traditionally eaten by the Australian Aboriginals.

But if ketosis is the therapeutic mechanism for treatment of seizures and bipolarity, this is not all that important. The keto side effects will be unavoidable. Much like the effects of carbamazepine, another treatment used for both conditions.