If the room is already built and you can’t run wires behind the walls (or more likely in the ceiling), then you should simply try out 2xFive+1xSub.  It’s sooo easy to just plug in and get on with your life…  If it turns out not loud enough to overcome the equipment noise, it’s also easy to get rid of, either as a gift or to another room, or sell on.  Truly there is no substitute for an in-home demo.  And you’ll hear any of the potential shortcomings -- see below -- and know how to choose either of your other two options thereafter.

Typical installation advice: in large spaces, go with more speakers but lower volume -- and in mono!  Most gyms have lots of echo, making that even more important.  20x23x13 is borderline, that’s only a quarter of a half-court, but still I’d go for 4 speakers not 2.  Floorstanding speakers might get nicked & scratched after a while, and toppled periodically; if you look at your local plebes’ gym you’ll notice speakers are high up on walls or ceiling.  Maybe as a solo upscale gym you won’t have their problems.  Exception: If hip-hop is your thing, floorstanding speakers do have the bass.  Otherwise a sub is required, not optional -- in any system, whether Sonos speakers or a Port with separate amp.  I love my Kef LS50’s, but they absolutely need a sub -- I use two cheap 8” DefTech “sub” (so-called) woofers, crappy scrubbable vinyl covering, good for a gym.

Easy way, 2 Fives in opposing corners and see how you like the sound.

Maybe add two more Fives in the other corners to even out the sound field and boost the low end a bit?

Alternately add a Sub to boost the low end, but not help level the volume across the room.

Adding a second (or more) Sub if you are happy with all but the low frequencies is possible.

You can fool with pairing the Fives in stereo or using them as singles. I’d lean to single and a wider sound field over paired and a smaller stereo image.

Port, and non-Sonos Amp and speakers is a huge question and you really need to listen to your options for speakers. I love the Klipsch sound, other folks not so much. A good quality 200/400 watt x 4 amp and one of these in each corner would tempt me:

https://www.klipsch.com/products/heresy-iv-floorstanding-speaker

Thanks. If I went with (4) Kef LS50’s or one of the higher end Kef Q models, what would you recommend I push that with in terms of an integrated amp? And could that amp also push a separate subwoofer? And I assume I can use the Sonos Port as input to this system…

Yes, you would use the Sonos Port as an input to your separate amp.  You could get a receiver with 2 zones (A/B speakers) and a subwoofer-out line-level RCA connector and configure it, or you could get a sub that took both line-level and speaker-level input and passed-thru the speaker-level to the LS50s.  That’s what I’m doing with 2xLS50, Definitive Technology ProSub 800, and an old Carver “cube” M-400t amplifier.  (I don’t recommend either product, by the way --  the DefTech sub amps fail too soon, and the Carver is a hold-out from the late 20th century!)  

Any amplifier recommendation I made would be talking thru my hat, but if you are intent on going this route, the topic has been forum’ed to death, just search.  Why did everybody obsess over this?  Because the LS50’s are slightly inefficient, and they have an impedance that goes way low in part of its range.  (Note this is why, though it is theoretically possible, I did not recommend using one Sonos Amp to drive 4 KEF speakers, even though it can do so with 8-ohm nominal speakers.  At the volume levels you need to overcome machine noise, plus hip-hop bass power, the Sonos Amp driving 4 speakers would likely limit its output (or maybe shutdown, though my understanding is that Sonos designed it to not fry itself).