As far as I know, gang wars can only be started is the usual way, by getting a "3rd kill" on a Vago or Balla when CJ is standing in a turf controlled by either Vagos or Ballas. However, once the gang war has been provoked it is possible to change which gang CJ will fight.
0x96ab64 // current gang war stage PC
= 0: no war
= 1: war provoked
= 2: first wave
= 3: first wave passed
= 4: second wave
= 5: second wave passed
= 6: third wave
0A8D: 0@ = read_memory 0x96ab64 size 4 virtual_protect 0
if 0@ == 1 then change the gang for the war
There are two groups that spawn during the war. The gang groups can be the same or different.
4@ = 4 // Da Nang
5@ = 5 // Mafia
0A8C: write_memory 0x96ab84 size 4 value 4@ virtual_protect 0 // Attack/War Gang
0A8C: write_memory 0x96ab50 size 4 value 5@ virtual_protect 0 // War Gang 2
Something else to mess with is the gang strength. This is usually determined by the density of the war zone. A gang strength of 3 is not seen in an unmodded game.
strength digits :
00 = armed with bats, 9mm pistols, Micro-SMG, and SMG ( rival density of 1 to 14 )
01 = armed with 9mm pistols, Micro-SMGs, SMG, and AK-47 ( rival density of 15 to 29 )
02 = armed with Micro-SMGs, SMGs, and AK-47s ( rival density of 30 and above )
03 = armed with SMGs, AK-47s, and Desert Eagles ( not seen without modification )
04 and above will eventually crash the game
0A8C: write_memory 0x96ab58 size 4 value 3 virtual_protect 0 // Gang Strength