Keeping an eye on the gang was a dangerous adventure.
Literal
Gang [object-を] watch [nominalizer-の] [topic-は] dangerous adventure was.
暴力団 is the official Japanese term for organized crime groups — synonymous with yakuza in English but more official/policing-flavored. 見張る ('to watch, to monitor, to keep guard') is a compound: 見る + 張る ('to stretch [eyes toward]'). A surveillance scenario.