[Konstantinos]

pawn Код:

COMMAND:gangs(playerid, params[])
{
    new str[200];
    for(new i = 1; i < MAX_GANGS; i++) format(str, sizeof(str), "%sGang Name: %s, Leader: %s, Members: %d.\n", str, GangStat[i][GangName], GangStat[i][Leader], GangStat[i][Members]);
    ShowPlayerDialog(...); // modify the arguments and "str" holds the info
    return 1;
}

by the way, why do you start from index 1 and 0 goes wasted? Indexes in arrays start from 0.