With a 4-year deal, Gaines will be gone when Tre's would kick in, and they'd probably draft a couple over that period anyway.
I think their dilemma is how they will get their QB? If they go for Cousins, they they have to fill their holes via the draft; if they move up to draft someone, then they will need to fill holes in FA. I think they'll go the latter route. They'll probably have a little over $30 million to play with. They should structure the bigger deals to reduce the hit this year (leaving more cash available for players this year), loading up in year 2 when they get the dead cap relief (over $20 mil). For example, say they could get Gaines with a 4-year $32 mill deal, which includes a $12 mil signing bonus. They could structure like so: ($3 + $3), ($3 + $7), ($3 + $5), and the same in year 4. I'd do something like this with their top 3 FAs. Just an example, but I think that dead cap space gives them some flexibility to add more players this year and smooth out the cap hit in the years after 2.