Hi @Athirah :
Do you want to add attachments through Gallery?
If so,this is impossible in theory, but I have a alternative solution.
The main point of this solution is to submit attachments through an invisible form. I've made a test for your reference:
1\My data source:

2\Add an edit form control(Form1)
DataSource
BookLog
Item
Gallery4.Selected /*I'll create Gallery4 later*/
3\Add a gallery control
Items
BookLog
4\Unlock Form1's Attachment data card and copy the Attachment control(DataCardValue3) into Gallery4(DataCardValue3_1)

5\Set DataCardValue3_1
Items
ThisItem.Attachments
OnAddFile
ClearCollect(TheActh,Self.Attachments);SubmitForm(Form1); /*TheActh is my custom variable*/
6\Set DataCardValue3 (which in form1) 's Items property to
TheActh
7\Set Form1‘s Visible property to
false

Best Regards,
Bof