What kind of guttering? Is it an aluminium greenhouse?, with a profile section to catch the runoff? Generally these have a moulded plastic block/endcap with the facility to attach a crystal pvc hose. If so, simply run a length of such tube across the end without the door to meet the outflow of the nearer side. You would have to allow the gradient to be sufficiently steep to keep it clear from debris.This will require the catchment barrel to be lower than otherwise, maybe preventing getting a can below the tap. Another thought may be to sink a barrel into the floor at the end of the path within the greenhouse. This could be especially useful to have water where you likely use it. Small solar pumps could lead onto an automated irrigation system, the possibilities are endless.