Hi @Oskarkuus ,
Could you tell me whether you have problem about displaying attachments in your gallery or updating your attachments?
1)how to display attachments in your gallery
If one item only has one attachment, you could set like this:
gallery's Items:
listname
image control's Image:
First(ThisItem.Attachments).AbsoluteUri
Just like this:

If one item has multiple attachments , you could set like this:
gallery1's Items:
listname
gallery2's Items:
Gallery1.Selected.Attachments
Image control's Image (inside gallery2):
ThisItem.AbsoluteUri
Just like this:

2)How to update attachments?
I'm afraid it's not supported to update attachments by using add picture button.
You could only use attachments control to update attachments. What's more, attachments only work inside a form and should be updated by using SubmitForm() function.
You should set like this:
gallery's Items:
listname
insert a form to upload attachments
set the form's data source to the same list
set the form's Item:
Gallery1.Selected
Only choose attachments in this form
set the attachments control's OnAddFile:
SubmitForm(Form2)
set the attachments control's OnRemoveFile:
SubmitForm(Form2)
Then after you select item in the gallery, you could upload file in that form.
Just like this:

Best regards,