I have received a lot of questions about flashes for the Fujifilm GFX 50S lately and HSS. It’s important to realize that the GFX operates with Fujifilm compatible flashes exactly like the X-T2 does, which is probably why Fujifilm recently updated their E-FX500 firmware update announcement. There are no reported instances of incompatibility, so if it works on X-T2 it should work on the GFX.
There also hasn’t been a lot of E-FX500 coverage compared to flashes like the Godox system or Nissin, which is probably because the EF-X500 is a little expensive and lacks radio communication. If you are really concerned about compatibility going forward Fujifilms E-FX500 system is probably the best investment, but Godox and Nissin have very compelling options today.
fotograficoweb – Fujifilm EF-X500 Shoe Mount Flash Review – Part One
fujifilm-x – EF-X500 testing on location Pt.2 by Max De Martino
fujifilm-x – EF-X500 Testing on location
Fujifilm EF-X500: B&H Photo/Amazon/Adorama