Use a file manager (
ES File Explorer,
Total Commander). Make the folders you want. Copy the picture files to the appropriate folders (cvopy, don't move - if a move goes bad you could lose the file). If the file gets copied properly, delete the original. (If there's a problem, the original is still there.) (BTW, all your pictures should be backed up to at least a computer [desktop or laptop] if you have one and a cloud account, or 2 cloud accounts. All important files should be. It's difficult recovering files if the phone gets driven over or stolen.)
Now run Gallery and you'll find the pictures in folders named for the last folder in the tree that the file is in. (IOW, if a picture file is in /DCIM/Camera/Vacations/Canada, the folder in Gallery will be Canada. So be careful that you don't also have /DCIM/Camera/Lakes/Canada, because they'll be in a folder named Canada too - you'll have 2 folders in Gallery with the same name.)