sadly this ticket seems impossible to price out stopovers. while TUN-HKG on RJ can have stopovers in both AMM and BKK.
also I personally have no qualms with Gotogate. It is fine for what it does, and one time it proactively offered me refund with no fee when airline cancelled my flights. Additionally, it is the one of the last standing bastions to book advanced tickets such as mixed cabin 3x and complex itineraries with scripts from Matrix, so I appreciate them for not torpedoing their (pretty lackluster) system like Priceline. But I know I am in minority here, as once I make a booking, especially special promotions and VERY special promotions, I do not aim to change anything at all.