It has been confirmed that some antivirus/utility software that is trying to keep up to date the apps installed on the host system may mistakenly install Opera Desktop on top of Opera GX. Proper actions has been taken to prevent such situations in a future but for now it is recommended to configure any AV software to exclude Opera from such updates.
If Opera GX has already been overwritten by Opera Desktop, please do as follows:
delete any version subfolder from the Opera GX installation directory (you can see example of them on the image attached above)
download and install new copy of Opera GX and install it in that same place as previously. The user data is in most cases kept outside of Opera's program files so you should be able to recover the GX installation completely to the state from before the overwrite.