Hi Eddie_18,
You can follow below steps so that you can move your files from gallery to SD Card :
1.) FILE MANAGER
2.) PHONE STORAGE
3.) Open the folder where you save your files then tap and hold it.
4.) You will that the file/folder has been selected
5.) Tap the Paper with a scissor on top like button.
6.) Go back to FILE MANAGER
7.) Go to SD CARD
8.) Tap the Paper Like option to paste the file you want to transfer.
Hope this helps!