I guess this is more academic (and will require practical verification by lau chiaus in the know) but for the benefit of interested parties:
List of gangs and places they (still?) own in Singapore:
From 21 :
Ang Meng Tong
Di Ang Ying 
Ang Yee Tong
Tiong Yee Tong
Pa Hai Tong
Ghee Meng Tong
Ghee Hin Kongsi
Ghee Hock (Part of Ghee Hin Kongsi)
Ang Guat Hwei 
Ang Tian San 
Feng Huang Shan (Yishun/ang mo kio)
Feng Qing Shan 
Guat San Sia (Known as moon sect few years back)
Deet San Sia 
Leng Heng Sia 
Yee Pak Hai 
Yee Chuan San 
Yee Chuan Hai 
Yee Hup Ho 
Tiong Meng Ko 
Heng Kee 
Gnoi Seek Kee 
Hup Ho Tong 
21 San[213] 
21 Hai 
21 Ang Hai 
21 Re Yue Ban 
Ang Soon Tong (Sembawang but inactive)
From 369(Salakau/Sio kun tong) :
Kallang Airport (Kallang)
Tanglin Halt 
Mei Ling Lor 
Queenstown Chao (Obvious)
Tanjong Rhu (Raffles place? (Not confirmed))(Quite active)
"Lak Ko"- 6th Avenue 
"Chit Ko"- 7th Avenue 
Taman Selasih 
"Goh Cho Bay"- Rochor Bay 
Teck Whye Lane (Choa Chu kang)
Yew Tee 
Kim Boon Tai 
Toa Payoh Chao (Obvious) (Quite active)
Marsiling Chao (Obvious)
Tampines Chao (Obvious)
"Ang San"- Redhill (Obvious)
Hougang Chao (Obvious)
Chao Lak Ho 
Sin Ming Lor/Sin Ming Road (Bishan) (Quite active)
Anak 537 (Tampines, Bedok, pasir ris) 
From 18 (zhup puay siao) :
Sio Yi Ho (Most active)
Hup Soon Heng 
Siao Kim Tian 
Ar Bak Kuay (Quite active)
Aljunied Cresent Siao (Obvious)
Toa Payoh Siao (Obvious)
Bugis Siao (Obvious)
Kranji Siao (Obvious)
3 Beh Lor 
4 Beh Lor 
7 Beh Lor 
Rochor Siao (Obvious)
Bedok Siao (Obvious)
Depot Siao
Purmei Siao 
Hougang Siao (Obvious)
Geylang Siao (Obvious)
Liew Lian Ka 
Telok Blangah Cresent Siao 
Tampines Siao (Obvious)
Tekka Siao 
Mai Siao Siao (Most inactive)
Sio Ka Po 
From 04 Gang : (Inactive)
Wah Kee (华记) 
China Town 04 
From 24 (Ji Si) :
Ghee Hai Kim (义海金) (Most people)
Ghee Lian Hor (Active)
Ghee Leng Kiat 
Patt Leong 
Sio Ang Kun (Most little people)(Bedok, Ang mo kio)(Just re-created cause it was inactive)
Sio Lo Kuan (Active)(Deskar road) 
08 (Kong Pui) :
Hai Lok San (108) (海陆山)(Old gang)
Hai Lok You 
Pek Kim Leng (白金龙) 
Pek Ka Long 
Kim Leng Hor (金龙虎) 
Sio Leng Hor (小龙虎) 
Di Chet (27)(Inactive)
Go Hock San 
Go Kong Go (505)(Inactive)
Sio Ang Leng 
Leng Hor Shan 
Sio Oh Leng 
Pui Kong Pui(808) 
Sio Pa Leng San 
Sa Kong Sa (303)(Quite Active)(Teck whye lane)
There are 2 other gangs :
Sarajumbo(Indian Gang)
 
Omega(Malay Gang)(Hangs out at Downtown East)
.