Can you use a unit with ganking at a battlefield after a showdown?

Yes, a unit with Ganking can move to another battlefield after participating in a showdown, provided the unit is ready (unexhausted).

According to RiftJudge FAQ #1961:

"Ruling: If a unit with Ganking is ready (unexhausted) after a showdown, you can use the Standard Move Action to move it to another battlefield during your turn. There does not need to be an enemy unit at the destination battlefield."

Important context for this interaction:

• Showdowns do not exhaust units: Participating in a showdown does not automatically exhaust your units, meaning they remain ready to perform a Standard Move after the showdown concludes, provided they weren't exhausted for another cost earlier in the turn.
• Timing: You must perform this move during your Main Phase, while the game is in an Open State (there is no chain currently resolving, and no active showdown or combat ongoing at that specific location).
• Standard Move restriction: You cannot move units using Ganking (or any Standard Move) while a showdown is currently in progress (Rule 143.1.c). You must wait until the showdown has concluded.