Gauss's law equates the total charge to the flux of the electric field by $\int{ \vec{E} \cdot d\vec{A}}=\frac{Q_{enclosed}}{\epsilon_0}$. This sequences develops the essential components of using Gauss's law: making symmetry arguments, calculating total charge, and using a flux integral to find the electric field.
Gauss's law can be used with charge densities which have high symmetry, however, some students have not developed the skills to make coherent symmetry arguments in introductory courses. In Paradigms, students begin to make explicit symmetry arguments to simplify their calculations in Gauss's law by using Proof by Contradiction. In Proof by Contradiction, the opposite of what you are trying to prove is assumed and then arguments are made using this assumption until a statement is made that is clearly false. For example, an infinite sheet of uniform charge density has high symmetry and explicit symmetry arguments and application to Gauss's law are made here as an example.
Students may struggle with understanding what $Q_{enclosed}$ means in Gauss's law. Charge distributions can have high symmetry in rectangular, cylindrical, and spherical coordinates, so strong integration skills in each of those coordinate systems, in various dimensions, are essential. Additionally, charge distributions are described by various terms such as uniform and constant which many students may not be familiar.
Gauss's law directly relates the flux of an electric field with the enclosed charge. Most students are introduced to Gauss's law in introductory physics, however, many do not understand the mathematics of the flux integral. Therefore, developing understanding of flux as done in the Geometry of Flux can assist students in properly using and understanding Gauss's law.