On the likely roster for opening day are the following players with contracts that expire or are eligible for free agency after 2024:
Corbin ($35.42 million)
Williams ($7 million)
Robles ($2.65 million)
Floro ($2.25 millon)
That's $47 million off the books.
Gallo has a mutual option for $8 million with a $2.5 million buyout. It's unusual for mutual options to be exercised, but I could see that in this case if he's not traded. Figure it this way: If he's playing well, then he's moved at the deadline. If he's playing poorly, he gets cut and the Nats are out $2.5 million. But if he's playing acceptably and perhaps Crews / Wood / Hassell / Morales don't have more than 1 or 2 ready for MLB, then it might be in the Nats interest to have him back at an additional $5.5 million figuring they weren't going to get much better than that for a one year guy, and it might be in Gallo's interest to take it because he had already tested the market this year and found he wasn't likely to beat $5 million by much.
All of these guys would be trade bait if they are performing well. Corbin and Williams would be a little tougher to move than Gallo, Floro or Robles if they are playing well. Corbin we'd have to eat salary to get anyone to take him off the Nats hands.