Just buy the games as gift and tell her to activate them on her GOG account, if she didn't have one, tell her to create one and activate the games, like this nobody can block or delete the account. It's probably the safest way to do it

I certainly am old fashioned. She does not have a GOG account.

Yes you can. Even if it were somehow against the TOS, there is absolutely no way GOG could detect something like that and it's not like it's causing any harm/damages.

Just create an account with her name, with your email address and credit card.

Then when you are ready to gift it to her, change the email address to hers.
Then tell her she needs to change the credit card detail.

The only issue would be if you already have a GOG account, using that email and credit card ... or if you intend to do so later, where there could then be an issue.

EDIT
If your wife/partner is willing and they don't have a GOG account, then maybe use their email and credit card instead, temporarily.

Just buy a load (whatever amount you deem appropriate) of games, and send her the codes via email, then she can set up a GoG account to redeem them. A lot easier than swapping email accounts and/or card details to achieve pretty much the same result...

Creating an account is just a moment, for something like this I would simply choose to buy the games as gifts.
If you want to keep it a secret to surprise her, better use your own mail as the recipient of the gifts and then give her the bunch of codes for Christmas.

if you make an email account then she has to use that witch is at best 1 extra account she has to keep forever... changing it over to her real account later isn't n option

you are best to use her account i.e, her normal email and then gift the games because that is less stress to maintain and fix any issues later

also useing the same router in the house will not be a problem and Gog does not check isp... if they did we wouldn't have 1/2 the spam we currently get but what will be a problem is both of you using Galaxy at the same time to download and play your games so even you do have good service its a good idea to limit Galaxy download speed on both systems to 1/2 whatever your download speed in mb

p.s, good luck