I like Gill's ideas Rich. Perhaps they could be involved in a number of different criminal activities because criminals, especially organized ones, often are.
For instance any individual, group, or organization that would involve themselves in a protection racket and/or a blackmail scheme would be likely involved in a whole host of criminal activities, anything profitable enough to warrant interest.
A related set of activities might include smuggling, extortion (not just for blackmail in the typical sense, but for favors, influence, etc.), shell companies (merchant activities and shipping companies) and fronting schemes, etc.
Another thing they might be involved in would be allowing or covering for an espionage or spy ring. Something that would be dangerous to the authorities but that an organization like the GS might have ties with. For instance a bazaar would naturally both attract and give cover to (foreign merchants would abound) spies. The GS might have ties with such spies, or even spy groups, maybe even cover them, and then use "fake extortion rackets" so that both groups, the GS, and the spies, tend to benefit in order to finance their operations.
A spy becomes involved with a Khanduq merchant which the GS then says they know about. (They know about it cause they secretly helped plant the initial contacts through a third party or a disguised member of their own party.)
The spy pays money to the merchant to let him operate but the GS then blackmails the merchant threatening to send him up to the authorities. The GS uses the blackmail to recover the money the spy has paid, thus reimbursing the spy while still allowing him to operate, and they blackmail for more than the spy paid in order to cover their own cut of the racket. the merchant is out money for both but is unaware of the relationship between the spy or spies and the GS.
This could lead to a lot of interesting role play opportunities. For instance the merchant or merchants could hire their own "protection" or even spies or undercover operatives to track their enemies and discover what is really going on.
Very nice pics of Istanbul by the way, especially the Hippodrome, the Cistern, and the Blue Mosque.
My own setting is very
Constantinople oriented (on our world anyway) and so I liked your photos.