Hi, @Gery.
Anybody’s copy of Core can watch any public wallet address.
What you need is the private keys (privkeys) from the copy of Core where that wallet was actually created.
It’s not actually possible to create a “watch only” wallet in Core. Core can only create a full wallet, with private keys and a public address. The public wallet address is created from the private keys, and could not be created without them.
Now, if you create a wallet with Core, and then go to a second installation of Core, you can import the public wallet address. Then the second installation of Core is then only “watching” that address, because it did not create that address and does not know the private keys. (You can also do this with anyone else’s public wallet address - any public address can be imported and watched, but that gives you no power over those coins.)
To gain full control of those coins on the second installation of Core, you need to import the private keys (privkeys) for that wallet.
If you have the original wallet.dat file from the original installation of Core where the address was created (or if you have a backup of the wallet.dat file), you can use that. The private keys are inside.
To use a wallet.dat file, you simply copy it into your .bitcoingold folder, but before you do this, make sure there are no coins being controlled by the current wallet.dat file. If there are, then make sure to first make a backup copy of the current wallet.dat file before copying in the old one.
If you have the private keys, themselves, you can import them using this command:
importprivkey "privkey" ( "label" ) ( rescan )
Adds a private key (as returned by dumpprivkey) to your wallet.
Requires a new wallet backup.
Hint: use importmulti to import more than one private key.
This will add those keys (from which it will know the corresponding public wallet address) to the current wallet file.
You can get your privkeys out of the original installation of Core (where the wallet was created) by using this command:
dumpprivkey "address"
Reveals the private key corresponding to 'address'.
Then the importprivkey can be used with this output
So, how can you have a wallet that says “watch-only?” You must have imported the public wallet address with this command:
importaddress "address" ( "label" rescan p2sh )
Adds an address or script (in hex) that can be watched as if it were in your wallet
but cannot be used to spend. Requires a new wallet backup.
Note that without the private keys, the funds cannot be controlled, only watched. You can import any public address this way.
Again, you cannot use Core to create a watch-only wallet. Creating a wallet starts with the privkey, and the public address is made from the privkey - in the original Core where the wallet was created, there must be a privkey inside.