There are two modders actively working on GCam for the ZenFone 6: Arnova8G2 and Savitar. Versions from other devs might also work, but these two own the phone and are able to do more about the issues.
If there’s a “configs” link next to the apk, check it and use one of the provided configs.
Android 10 »
Avoid upgrading to Android 10 if you want to use Google Camera. Too many issues at the moment.
- GCam_4.0.191101.0729build-7.0.009_x9_s9s_mod_0.42.apk (xenius9, 2019-11-13, changelog)
With this config file: zf6_ax_beta.xml