From Homestar Runner Wiki
Gavin is a cockroach who enjoys crawling on Strong Bad's computer screen occasionally. He debuted in an Easter egg in the email personal favorites, and has made infrequent cameo appearances in later Strong Bad Emails and Toons. Strong Bad seems to like the little guy, although he felt upstaged by the insect in Experimental Film, saying that Gavin got as much screen time as he did.
Another cockroach was seen in Strong Bad Main Page, long before Gavin's name was revealed in "personal favorites". Whether it is Gavin or not is uncertain. A large swarm of roaches, similar in appearance to Gavin, appeared in diorama. He was also seen in the email looking old, crawling over an "eStrong- Vague Online Investments" box.