You don't absolutely need any gateway at all, but you'll want one. There are several existing threads with recommendations. I ended up using a refurb HP nettop/thin client as the gateway. The hardware just needs to be fast enough. No special settings needed. If you use OpenWrt like me, the following four hardware targets are fast enough as of this writing:
mt7621 (used in various specific recent models from Netgear, Linksys, D-Link, TP-Link, Ubiquiti, and several Chinese brands)
mt7623 (used so far only in the Banana Pi R2)
mvebu (mostly Linksys WRT-AC models)
x86 (assuming you don't try to use, for example, something from the previous century or equivalent)
The rest will be too slow. The latest generation of processors by Qualcomm should be fast enough too, but they are not yet supported.