Both don't have a whole lot of value due to their contracts.
Gordon comes to 9.2 WAR value, but he's owed $13.8M with a club option for $14M in 2021. This knocks him into negative territory (-4.6). He's age 31 and his best asset (his legs) are only going to get worse from here on out coupled with his dwindling on-base numbers.
Seager comes to a 19.1 WAR value, but he's owed $38M over the next two years (club option for $15M in his age-34 season in 2022). He carries basically no value (0.2 value). You might find a 3B needy team at the deadline and unload him because of his pop from the left side and average defense.
You'll most likely need to attach a prospect to get any sort of value,
but that kind of defeats the purpose of a rebuild. What position(s) are you looking for?