Assuming that your greenhouse is built to give fall to the chosen end, then you need a brass hose union barbed piece*, a small pack of epoxy paste(Such as Repair Metal for Good), a drill bit of size to suit (1/2"), a sufficient length of 1/2" hose, a Jubilee clip.
Drill a hole about 2" from the low end of the gutter, from the top surface. File a slot across the hose union threaded piece to allow water to enter the spigot. Bed the union into the hole from the top with the epoxy paste. Form a ball of a second dollop of the mixed paste of sufficient size to fill the end of the gutter at (both) ends to prevent leakage. When set, attach the length of hose to fixed spigot and apply clip carefully.Place the other end directly into butt. This will enable the gutter to fill under rainfall, and then trickle into the pipe and thence into the butt. Keep fitting clear of debris.
*https://www.bes.co.uk/screwed-tail-adaptor-brass-1-2-bsp-x-13mm-hose-bore-17546/