As for the gain, the Lyapunov stability analysis gives you requirement for the selection of the gain. Upper-bounds of uncertain elements are key component of the gain function. For more information, you can refer to "Applied Non-linear Control" by Slotin.

In case of conventional sliding mode controller however Lyapunov stability gives you information about the gain but you should to know that in practical application you should to defined it by optimization. For more information you can refer to "sliding mode control hand book" or some papers which published by our research team.