It is a more than valid question
I'd just use it as a frame/wheelset and go 1*11: SRAM Rival 1 brifters (136€), Rival 1 rear derailleur (85€, better seals than Apex), a chinese crankset (love the zrace stuff, 750gr incl. BB: 70€) and cassette (11-46), either mini-v or (If you have disc brakes) just give it a try with the current brakes.
Even cheaper would be to go all chinese: get a sensah grx set for 220€, brifters, rear derailleur, cassette, chain, crankset and bottom bracket included, add a nice drop bar (i want to try the Rose gravel thing, the only one i know with front sweep) and some bar tape (Decathlon, 6€ or ordered from China).
If you are able to mount it on your own it's 270-360€, still cheaper than to get a new gravel bike and a lot of these fitness bikes have a gravel/endurance road geometry, esp. Serious used the same frame as their Roubaix bikes for the Sirrus in some years.