The term "gross" refers to something that is obscenely large.
To generalize means to describe things or people in terms of a group rather than individually.
Saying Americans are wealthy is a generalization because, though there plenty of poor or middle class Americans, as a group they are considered wealthy by many people from other countries.
This an example of a gross generalization because of the number of people being included in this group of supposedly wealthy people.
The generalization is large but what makes it obscenely large (or gross) is that many Americans would find that assumption both ignorant and offensive. Demonstrating how gross generalizations usually have negative connotations.