Cleanup

• You should really use better variable names.
  • Child is a poor name for a Gate.
  • Object is a poor name for a Player.
  • GetDistance is a poor name for something that doesn't return a number. IsNearGate or NearGate makes more sense.
• You don't need parenthesis around conditions, and it's not idiomatic to include them.
• It's widely regarded best to fail fast. There's no reason for GetDistance/NearGate to only continue if it's given a player. The only check that might belong is assert(Player:IsA("Player")). It is the caller's responsibility to give the right parameters.

The Problem

There are two problems with this code.

First, that only the first torso will be tracked by this script -- if they die, you'll still have Torso pointing to the first torso they had. This actually won't matter, because of the second point:

.Changed doesn't fire for changes caused by physics. Consequently, while teleporting the player will trigger the changed event, walking won't.

There isn't really a good event to get when the player moves (it would happen too often anyway). It's much more straightforward to just check every half a second with a while loop:

Players.PlayerAdded:connect(function(Player)
	while Player.Parent do
		if NearGate(Player) then
			print(Player.Name .. " is close to a gate")
		end
		-- check twice a second
		wait(1/2)
	end
end)

Note that DistanceFromCharacter returns 0 if the player doesn't have a head, so between lives this will believe that they are near the gate. It might be a good idea to require Distance to also be above, say, .01 in addition to being less than RequiredDistance