// You should have some variable which creates the gate. I have given a sample as Gate1

new Gate1 = CreateObject(objectid, X, Y, Z); // Variable Gate1 has given value of creating a gate.

if (strcmp("/opengate", cmdtext, true, 10) == 0)
    {
    if(IsPlayerInRangeOfPoint(playerid, 15.0, X, Y, Z)) // the range till where player can use this command.
    {
    MoveObject(Gate1, X, Y, Z, Speed);  // Move the object to where it looks it's opened.
    SendClientMessage(playerid, 0xFAFAFAAA, "Gate opened");
    return 1;
        }
    }
    if (strcmp("/closegate", cmdtext, true, 10) == 0)
{
    if(IsPlayerInRangeOfPoint(playerid, X, Y, Z))
    {
    MoveObject(Gate1,X, Y, Z, Speed); //You will need to paste same coords as you make for Gate1 value.
    SendClientMessage(playerid, COLOR_GREEN, "Gate closed");
    return 1;
        }
    return 1;
}