You need to use onClientRender (this event is triggered on every frame) or a timer. Then on every frame get the label's position and set new position by taking away some pixels from X coord. Eg.:
addEventHandler( "onClientRender", getResourceRootElement( getThisResource( ) ),
function( )
local x,y = guiGetPosition( label, false )
guiSetPosition( label, x-10, y, false )
end
)
You'd have to remake this code a little bit, because this will keep moving the label even if it goes out of screen.