Can this work?
function MarkerHit --Function start + name
createMarker(314.20001220703, -133.69999694824, 998.59997558594, "arrow", 1, 0, 100, 125, 255) -- Create's the marker (but I don't know if it is in Interior 7 now?!)
setElementInterior(Player,7, 314.20001220703, -133.69999694824, 998.59997558594) -- sets the Player to Interior 7 on Marker hit (doesn't need to go to other location only open the GUI)
addEventHandler ( "onClientMarkerHit", openGUI, getRootElement(), MarkerHit ) -- on Marker hit opens the GUI