The International Phonetic Alphabet (IPA) symbol for a voiced velar stop is [ɡ], as in the English word ‘go’. The Americanist tradition of phonetic writing uses the symbol [g].
The IPA symbol for length of a phonetic segment is [ˑ]. Actually this symbol indicates half length. Fully long segments are indicated by [ː]. The Americanist symbol for length is middle dot [·], with the possibility of using a colon [:] for a greater amount of phonetic length.