Yeast
If you don't have a stirplate, you'll probably be wanting to think about getting one. If you're fermenting in a single fermentor or splitting your batch between 2, you're effectively also doubling the quantity of yeast you need to be pitching and stirplate and large enough vessel (at least one gallon) are a large help.
One of the things I love about 10 gallon batches is pitching two different types of yeast into the same wort. So, if you're a liquid yeast user like me, you'll want two stirplates for your starters.
Buying in Bulk
Figure out a way and just start doing it. The cost savings is fantastic. With 10 gallon batches, I'm blowing through grain and hops. But by buying in bulk, my cost (non-equipment) is generally less than $0.50 per 12 oz beer.
PUMP and Valves on all kettles
These things in conjunction are the two upgrades that have made my brew days effortless. I know you mentioned a gravity fed system, so you may not have to worry about it as much or at all. I didn't have that luxury, so a pump was necessary to pump my strike/sparge water and the wort into the kettles at the end of the boil. Gravity or not, a pump is essential if you want to recirculate your wort during chilling. I recirculate through my plate chiller (and many use a recirculating whirlpool immersion chiller) that really helps me chill and (with the plate chiller) allows the bulk of the volume of wort to come down quickly (rather than have the mass volume of wort in the kettle sit hot while it does one pass through my chiller into the fermentors).
More fermentors
Get these too.
Kegs
Also extremely helpful, although I did bottle a couple 10 gallon batches before I finally upgraded.