I am not sure why you would desire a gate valve in this scenario as opposed to a ball valve. The ball valve is less expensive and gives you on-off control. The extra cost of gate valves are for the fine flow control, which you should already have with the DC pump.
From your description, it sounds like you are talking about a valve between the overflow and the sump on the way down. Regardless of direction of flow, here is the general concept I try to follow. At some point you may want to disconnect one portion of the system for maintenance or repair; however, you don't want to lose water or need to pump out and store more than necessary. I try to use union/ball valves in my plumbing to facilitate this.
As to the location for this connection, I am not sure it matters much whether it is closer to the display or sump. It needs to be in a location that is accessible and allows you disconnect the system into pieces that are easily removed and re-installed.
Best of luck!
