The reason you got teleported when you walked into the gate is that there are some default "teleport" coordinates setup in UOX3/JS/teleport.scp which work independently of the actual moongate item and any scripts you might assign to it (such as the menu-based moongate-script).
Try the following: While UOX3 isn't running, back up teleport.scp, open the file in notepad and then delete any entry/line that contains the letter-combination "_mg_". Save the file, and start UOX3 again.
The "hardscripted" (heh) teleport coordinates at the moongate locations should now not be in effect, and you should no longer get teleported until you select a location from the menu.