I recently finished reading the first volume of The Feynman Lectures and he brings up the possibility that G may not be constant, that it may vary over time or space. The argument why G likely doesn't vary over time is that a different value of G in the past, either bigger or smaller, would be inconsistent with what we understand about the formation of the Earth. If it were bigger by even a small factor then the Earth would be closer to the Sun and hence too hot for oceans to form and vice-versa if it were smaller.
Based on all current observations, G is constant.