Sonic.net

Do I (really) need to rent a residential gateway (RG), in order to get Sonic fiber?
When Sonic fiber comes to the neighborhood, do I have the option of remaining on DSL, to avoid the gateway rental fee?

No.

Yes.

If no, how does one get around the rental gateway? What models of gateway work with what settings?

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.

I use a Lynksys just fine with fiber.

I don't believe you can keep DSL because Sonic would have to pay AT&T for use of the line. There's really no downside to fiber.

No you don't. For some reason they had to leave one with me when I got the fiber installed, but I called support a few weeks after and returned it when I was sure I didn't need it. I ended up being charged for one month of rental.

I use a Ubiquiti Edgerouter POE for what it's worth... get gigabit throughput on it.