The ranks used in the Drenai world aren't exactly clear and only a few are ever really mentioned and with so little context to make working out exactly what they may be equivalent to, a bit of a nightmare.
The explanations here are partly suppositions based on pulling together the little bits of information from various books in the series.
It is also important to remember that although modern ranks are mentioned here as a way of comparison, the Drenai army's organisational structure may be closer to the "ranks" and unit sizes of English armies in and around the 15th century (Ventenars, Centenars etc.), but again this is just guesswork.
A Gan is a commanding officer in overall charge of a force, equivalent to either a General (First Gan, perhaps matches this more accurately as Orrin is in charge of all forces at Dros Delnoch in Legend) or a Colonel (Gan Hogun is commander of The Legion, but answers to Orrin).
Appears to be an under-officer of sorts as Dun Pinar in Legend is described as effectively running the Dros under Gan Orrin (due to his perceived ineptitude). Possibly equivalent to a Major or Captain as they are shown as being in charge of at least 50 men (Legend) and are considered a commissioned rank.
Appears to be an uncommissioned rank, perhaps one of the lowest/entry-level ranks like a Lance-Corporal or a Private.