To enable Aux camera support on gcam, after a lot of hard work of devs and resarch of users, it has been discovered that the app should be installed in priv-app in order to get full elevated permissions. This can be achieved by using the magisk module "app systemizer" or as it was explained on this same thread:
"Multi-Lense GCam without SELinux Permissive
As it turns out, the multi-lense modded Gcam app requires elevated permissions to switch between the auxiliary back cameras. Note that these are not standard android permissions, but lower-level system permissions that generally cannot be enabled by a standard user app.
By rooting and setting SELinux to permissive, you allow the camera app to access these permissions, but you leave yourself with a rather large security vulnerability as now every app on your phone has access to these permissions. You can provide individual apps with these permissions simply by installing them as a priv-app in the systems app folder without setting the global SELinux status to permissive. This can be done either by manually moving the Gcam apk to the priv-app folder, or with a Magisk module; either way, you have to be rooted to do so at the moment.
Here are the steps I took to enable switching between the lenses without installing the NFS injector module or manually setting SELinux to permissive:
1) Install the previously shared multi-lense Gcam app as a standard user app by installing the apk as you would normally.
2) Open the camera app and enable the usual set of permissions (local storage, camera, etc.)
3) Install this Magisk module in Magisk or manually move the Gcam apk to /system/priv-app and reboot
The Magisk module attached simply creates a copy of the Gcam app and its included lib in the /system/priv-app folder and does not modify any other security setting. The Gcam app may crash when opening if you do not install it as a standard user app first and set it up before installing it as a system app (at least it did for me at first)."
Now once you have AUX access enabled, depending of the gcam version you are using, the next step is to know how to swich between lenses. If you are using the recommended gcam on this post, first i recommend to set the auxiliar switch mode to "long press". Then you will be able to switch between main and telephoto if you long press the arrows camera change icon. Single press this icon will cycle through front and previously used camera.
If you long press the photo preview icon on the viewfinder, the camera will change and start switching between main lens and wide angle. (Note that you may have this reversed, the important thing to know is that pressing the photo preview icon on the viewfinder will let you use the other secondary lens, and they will always will cycle with the main lens.)
If you are using the arnovas recent test apks you should be able to see a pine icon. Just click there to change lens. Bare in mind that this recent version are still up to bug correcting, so you may find unexpected results.
Recommended version works for both photo and video for each lens.
If you find grainy or noisy video while shooting video on medium to low light conditions on wide or tele, you need to lower exposure by tapping on screen and lowering exposure with the floating slider controller.
Thanks to Arnova and all the users on telegram testing out this magnificent work. Will keep posted if a rootless version of this method is found.