"Gozaru" is originally a courteous expression of "iru" or "aru" ("be") used at the end of a sentence. It was used in 14c. to 19c. As a role language in a fiction, it basically suggests that the speaker is samurai or ninja, with a personal pronoun of "sessha" or "soregashi" as "I" and other old-fashioned speech. For example, "Sessha wa Hattori Hanzoh de gozaru," meaning "I am Hanzoh Hattori." We don't believe a real ninja actually used such a word which easily indicated he is a ninja.
We can hear "gozaru" in some summer 2015 animes, e.g. Overlord, Kangoku Gakuen (Prison School), and Charlotte. Are you aware who uses this? It is Hamusuke (Hamsuke, the Virtuous King of the Forest) in Overlord, Gakuto in Prison School, and Ayumi Otosaka (Yu Otosaka's sister) in Charlotte. As you know, they are not samurai or ninja. Then what does their "gozaru" suggest?
Hamusuke was born several hundred years ago and is known as the strongest beast. So, her "soregashi" and "gozaru" may suggest she is old and strong like a samurai or ancient soldier.
Gakuto is a high school student who has a strong otaku interest in Three Kingdoms, a historical China story in AD 3c. Gakuto's language suggests that he devotes himself to the ancient world so much that he is using old-fashioned words. Of course, Chinese people did not use such Japanese.
Ayumi usually doesn't use "gozaru." She only uses "gozaru" in an intimate talk with her brother Yu. She has a strong interest in star watching and pop idle. I suppose her usage of "gozaru" would suggest she devotes herself to something very much, or she is a kind of otaku, like Gakuto.
Her "gozaru" reminds me Saori Bajeena in "Ore no imouto ga konna ni kawaii wake ga nai"("Oreimo"). Saori is a leader of an otaku group and uses "gozaru," but actually she is from a wealthy family and speaks formally when not with her otaku friends. Saori's "gozaru" is a part of her character buildings to be an otaku.
So, recently, it seems "
(Apr 19, 2016, corrected a typo "otaku" to "gozaru")