Yes, you can do it with a select field, of course:
title: Gallery
pages: false
files: true
options:
delete: false
fields:
gallery:
label: Image
type: structure
style: table
entry: >
{{gallery_image}}
{{caption}}
fields:
gallery_image:
label: Image
type: image
class:
label: Class
type: select
options:
one-third-offset: one-third-offset
half: half
...
caption:
label: Caption
type: text