Thanks a a lot Holly.
I think you can resolve this by switching your gallery “Link To” settings from “Attachment Page” to “Media File”. This should display your gallery images in a pop up slider format.
To change this setting click anywhere on your gallery content block and that should bring up a “pencil” icon towards the top of the editor. Click on the pencil icon to open your “Gallery Settings” window and change the “Link To” value from “Attachment File” to “Media File”. After that, update the gallery and then publish your page to review your changes.
I hope this helps. Please let us know if there’s anything else that we can answer for you!