I've purchased a couple of older-style Gitzo tripods in the past for repair whose top plates are either missing or are broken.
For replacement plates, this is available either as a two-part print or a single object.
The plate is originally Aluminium, and the cover is some sort of softer plastic.
For a two-part piece I have printed the plate itself in PETG and the cover in TPU. This has worked well so far and bonding them with a thin layer of epoxy resin.
For a single part replacement I would guess that PETG is a safe bet, especially as it is less prone to cracking on impact and threads work pretty well in this material.
The top cover has a detente into which the notch on the tripod slips, so that the head is secured for storage.
I have been using these prints for over a year now on my tripods and have had no issues, they have stood up incredibly well.
Top Plate
Plate Cover
The author hasn't provided the model origin yet.