If i can, the gallery is made for share your creations with everyone, if you want to take a private "backup" of your creation, i suggest to buy an usb-key (you don't need a bigger one, even a small 8gb usb key is good) and copy the tray folder (and if you want the save folder), you can find in the the sims 4 folder, in documents, but if you use mods, you just already know, the tray folder, is the folder when the data of all your creations are saved.
That´s an option, but it´s not an official way to do it, i still would like to be able to save my creations in a private backup, linked to my EA account
