For those of us studying, or just interested in, Okinawan history and culture, a lot of the standard resources can prove pretty hit or miss. Kotobank.jp, one of my favorite go-to online encyclopedia sites, for example, which will simultaneously search the Daijirin, Daijisen, Sekai Daihyakka jiten, Nihon jinmei daijiten, and a whole bunch of other Kodansha, Shogakukan, and Asahi Shinbunsha encyclopedias for you, gives quite a few hits for Gosamaru, but none for Nakagusuku udun.
Thus, for Okinawa topics, the Okinawa Compact Encyclopedia 沖縄コンパクト事典, published & provided online by the Ryukyu Shimpo (one of Okinawa’s oldest and top newspapers), has become my go-to source for basic information on major topics in Okinawan history and culture. I should point out that the whole site, and all the encyclopedia entries, are in Japanese, so your reading skills have to be up to snuff to make use of this.
With more than 3400 entries, it won’t cover absolutely anything you ever need to look up – I certainly myself have come up blank on a number of occasions – but, for many of the most prominent or significant topics, from the Heshikiya-Tomoyose Incident of 1734 to different types of tribute ships to songs like Aha Bushi to American military equipment, the Okinawa Compact Encyclopedia will provide you a nice brief but dense explanation, including pronunciation (reading) of the kanji, and in the case of biographies, birth and death dates, alternate names, and so forth. Terms included in the encyclopedia range from myth & legend to the premodern history of the Ryukyu Kingdom, to wartime & post-war topics, e.g. relating to the US military bases.
And it’s perfectly free to use. On the website, all the entries are listed on a single page in gojûon order (that is, あいうえお、かきくけこ). I often just use the search function in my browser to search by kanji, or by reading in kana.
If there is one thing I find just a little disappointing about the site, it is that it uses mostly standard Japanese readings, with only a smattering of Okinawan readings, rarely if ever providing the Okinawan reading for something more standardly known by its Japanese reading. For example, the encyclopedia lists 風水 (C: feng shui) with the Okinawan reading funshii (ふんしー, as opposed to the Japanese fuusui ふうすい), but gives Shuri castle as Shuri-jô, with no mention of it being Shuri gusuku or Shuri gushiku, let alone the more fully traditional Okinawan pronunciation of Shuri as Sui. And if you’re wondering whether the Okinawan for Nakagusuku should be Naakaa- or Naaka- or Nakaa-, you’re out of luck, and have to go rely on another site entirely, like Ryûdai’s Ryûkyû-go onsei database 琉球語音声データベース (turns out it’s just Nakagushiku, no long vowels).
Still, it’s a great resource. If your standard Japan encyclopedias are getting you down, take a look at the Okinawa Compact Encyclopedia. A great basic starting place for looking up Okinawa-related names, placenames, and terms.