Freebbble!

HUE Gradients: Free .sketch Kit is a free Various design resource made by Dribbble user Vladimir Kudinov. As far as I can tell, its license is Creative Commons.

Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.