Four ability characters need G12+0, all abilities omega'd but not zeta'd and if I recall correctly, Mk V-A mods (Gold but not 6 dotted).
Alternatively characters can get there with the zeta without being G12. Darth Revan and his three zetas make it very easy to get there from a gear perspective, but clearing the event with a low geared DR might be a challenge. It's a hard event.
The typical questions are usually "do I want to gear Juhani or Canderous." I went with Canderous and he's a staple on a reasonable GA / TW team. The Carth, Mission and Zalbaar zetas make that team go, so if you needed to prioritize gear over zeta, I would look to Canderous first.
The DR and JKR staples should either have their gear already or are easier to get to the 17,5 cut off because they have multiple uniques.