ActiveAdmin: Generating drop down list in form for has_many/belongs_to relationship

Change your form to the following

form do |f|
  f.inputs "Image Details" do
    f.input :gallery_id, as: :select, collection: Gallery.select(:name).uniq
    f.input :file
  end
end

Try this

form do |f|
  f.inputs "Image Details" do
    f.input :gallery
    f.input :file
  end
end