With a Beananimal overflow it's similar to a herbie but has a third drain which is a standard standpipe. The idea is to adjust the valve so you get a small amount of flow through the standpipe. This is easily achieved with a ball valve, I see no reason really to spring for a gate valve. If you want to fine tune it with a gate valve then simplify the overflow and go straight herbie with only two drains but IMO using a gate valve on a Beananimal kind defeats the purpose of that particular design.

I actually put slip-slip unions on either side of my gate valve. That bastard is expensive, so it's getting recycled through many future builds.

I wpuld go threaded, I just setup my Bean animal, and took photos, will be posting on my 187 gallon tank build later today