You can use the search features offered by the gem program directly, like so:
gem query --remote --name-matches class
Output:
*** REMOTE GEMS *** acts_as_wrapped_class (1.0.1) arrayclass (0.1.0) bkerley-template_class (0.0.2) change_class (1.0.0) chrislo-sourceclassifier (0.2.2) class_associations (0.0.5) classifier (1.3.1) classroom (0.0.2) classx (0.0.6) classx-pluggable (0.0.1) classy-inheritance (0.6.2) giraffesoft-classy_resources (0.3.0) johnsbrn-classy-inheritance (0.6.8) loe-iclassify-interface (1.0.0) magic_metaclass (1.0.0) shuber-eigenclass (1.0.1) unageanu-javaclass (0.2.1) yury-classifier (1.3.3)
Here, you asked the repository for all gems with names including the word "class."