How do I add a receptacle outside off the porch light
You can add a box extension with knockouts, then run conduit down to a box with a WR weather resistant GFCI. If the cover or roof of the gazebo protects the location from rain a standard cover can be used but if it is wet your will need a hard use or in use cover (and the box itself will need to be listed for wet locations). Some folks don't like surface mount conduit if this is you you can box the conduit in. I have seen some very artistic "cover up's of conduit that made it look like a grape vine at first I did not realise it was conduit as they were on several posts.