A person who collects gnomes can be referred to as a "gnome enthusiast" or simply a "gnome collector." While there isn't a specific term exclusively for gnome collectors, they often engage in this hobby due to a love for garden decor or folklore associated with gnomes. Collecting gnomes may also reflect an interest in whimsical or quirky home aesthetics.
A person who collects beetles is a coleopterist.
Bibliophile
supernatural
collecter
A cravatologist.
a strawber.
A car collector?
a button collector
A paper collector
A badge collector.
A gem collector
If you need the answer to the joke it is "a watchman"