Hadn't the DOJ already declined to prosecute him?

Possibly. I'm not immediately familiar with that detail of the history between Gaetz and the DOJ. But the still hidden ethics committee report could have changed that decision and prompted additional investigation and potentially prosecution. But I'm virtually certain that even reopening the previous investigation won't happen under the Trump administration.

Unless Gaetz fails Trump in some way (see, for example, Cohen, Guiliani, whats-her-name Kraken, and multiple others over the years). He will protect his people only while they remain loyal and useful to him. For the moment, Gaetz appears to still be loyal. Usefulness is a different question. There may be some other position in the administration that he could fill - preferably one that doesn't require advice and consent.

--Peter