Well I think it's more likely they re-sign Eflin than Gibson, but sure, they might have to replace both, or they could try to bring Gibson back on a one-year deal.
Heck, Ranger is no sure thing either! And if they acquire someone with more than one year of control (not including this one) Nola's spot will be opening as well.
But on paper, Eflin still has an option (albeit one that is not likely to be executed) and a longer future ahead of him while Gibson is a free agent. And I'm assuming they might acquire someone Gibson-like (i.e. a 4th/5th starter with one year of control) rather than someone younger/better/more controllable.
But that is what make it interesting in terms of who DD might pursue and what it costs. The cost will be determined by the guy's service time or contract status as much as his talent.