If you really want to write such a general statement: Look at the statement and think about what it means. The references should provide evidence that the statement is indeed true. So in your example, you need a selection of references that show that "Gaussians are widely used in mathematics & statistics" (sorry, it is as simple as that). This selection does not need to be complete, nor must it be 100% representative. A few more or less random, but fitting references from publications using Gaussians within the mentioned topics suffice, maybe from different time periods and different (well-known) authors. I would go against citing reviews only, at least a few primary sources should be mentioned.
However, it makes more sense to be more specific and to give some more context. For example you could simply write:
Gaussians are widely used in mathematics & statistics, e.g. in topic 1 [citation 1], topic 2 [citation 2] and topic 3 [citation 3].
In this case, the selection of references is a little less random which is to be preferred.
Your example is not a good one, it is common knowledge and does not require citation. However, I treated it just as an example for a general statement, and of course there are general statements that are not common knowledge.