Hidden trackable information in games sucks!
I play games for fun and pursuing the goal to win the game is fun! :)
"Gang" card say : "Build town walls without entering Templum".
Must i pay 1 marble or not for this town walls?
Because for other cards as "Architect", we say we must pay 6 marbles.
Both with Gang and with Architect, you must pay marble as usual. It explicitly says so in brackets on Architect. While it doesn't explicitly say so on Gang, that isn't really necessary. The card only tells you that you can build the town walls without entering Templum. If it also let you build them without paying marble, it would need to explicitly say so.
This is also how it is programmed in the online version of the game at Yucata.