1. Choose a "strict" name for your gang that has no special characters or whitespaces (you can still use those later) like in the "Name" field explanation (step 4).
2. Create (or copy already existing) json file and name it: <YourStrictGangName>.json (without <> e.g "DevilsHorns.json")
3. Copy the structure below and place it into the .json file you created:
{
  "Name": "StrictGangName",
  "Alias": "Custom Alias",
  "Hash": 0,
  "HudColor": 21,
  "blipColor": 50,
  "MemberVoicePool": ["G_M_Y_BALLAEAST_01_BLACK_FULL_01", "G_M_Y_BALLAEAST_01_BLACK_FULL_02"],
  "MembersPool": ["BallaEast01GMY", "BallaOrig01GMY", "BallaSout01GMY", "Ballas01GFY"],
  "VehiclesPool": ["Voodoo", "Tahoma", "Greenwood", "Buccaneer2", "Blade", "Cavalcade", "Cavalcade2", "Granger2", "Virgo3", "Faction", "Chino2"],
  "VehicleColors": ["MetallicBlack", "BrushedBlackSteel", "MetallicBlackSteel", "MetallicCream", "MetallicWhite", "MetallicMidnightSilver", "MattePurple", "MetallicPurple", "MetallicBlackPurple"],
  "VehicleColorKeyword": "",
  "Drugs": ["Crack", "Marijuana"],
  "ForcePedVoice": false,
  "Relationships": {
    "Armenian": "Neutral",
    "Ballas": "Respect",
    "Families": "Neutral",
    "Kkangpae": "Neutral",
    "Lost": "Neutral",
    "Madrazo": "Neutral",
    "Marabunta": "Neutral",
    "Rednecks": "Neutral",
    "Triad": "Neutral",
    "Vagos": "Neutral",
    "Varrios": "Neutral"
  }
}

4. Explanation of each attribute:
Name: You can use any gang name you like without whitespace/special characters (e.g "DevilHorns")
Alias: Any name you want with no restrictions (e.g "Devil's Horns")
Hash: Leave 0 for custom gangs
HudColor: List of hud colors: https://docs.fivem.net/docs/game-references/hud-colors/ make sure to copy from the "Index" column
blipColor: List of blip colors: https://docs.fivem.net/docs/game-references/blips/#blip-colors make sure to use number
MemberVoicePool: I usually use Menyoo (or other trainers) to check ped voices. But you can also see the list here: https://pastebin.com/vDWd8RYx
MembersPool: List of ped models to spawn as gang members; Full list: https://gtahash.ru/skins/
VehiclesPool: You can use menyoo when spawning vehicle it will show the "ModelName:". Also you can use list: https://gtahash.ru/car/
VehicleColors: List: https://wiki.rage.mp/wiki/Vehicle_Colors (Make sure to remove spaces between words [e.g "Metallic Gasoline Blue Green" => "MetallicGasolineBlueGreen"])
VehicleColorKeyword: Please leave empty for custom gangs
Drugs: Drugs that this gang deals. Available options (Marijuana, Crack, Cocaine, Meth, Heroin)
ForcePedVoice: If set to true spawned members will be assigned random voice from MemberVoicePool. If you use peds that already have voices (Like default ballas peds) you can leave this as false.
Relationships: Relationships with other gangs; Available options: Respect, Like, Neutral, Dislike, Hate
(!) Important: Make sure to add your strict gang name to "Relationships" to make it respect itself