Hey Daniel,
Thanks for reaching out!
To achieve your goal in your single layout, we need to duplicate the rows:
Then we can check the number of your gallery by using this dynamic content: {{dc:looper:count}}
Then in the first row > column, I added a condition {{dc:looper:count}} == 1
, which means if the looper count is only 1 item, this will display the gallery into 1 column.
Then in the second row > column, I added a condition {{dc:looper:count}} > 1
, which means if the looper count is greater than 1 item, this will display the gallery into 2 columns.
Please note that I have implemented the changes to your Gallery Single Layout.
Hope that helps.