I'm trying to prevent the resizing of the individual images but can't seem to do it correctly.
I have 557 images that are 50 to 262 pixels wide and all are 120 pixels tall. When I do the montage it still resizes some to be smaller. Centering / scale is set to 0.5 which I thought prevented scaling but it doesn't seem to. Set montage type to "auto" none of the images are still 120px tall. Some are down to 3px tall but all have been reduced at some point. Changing from auto to horizontal or vertical array generates a "too many input images" preview error and the same if I try to apply/ok. Merging mode is set to scaled but if I change it to aligned they don't stay aligned. Auto displays them but breaks them up however the H and V array options still give the same error message.

Mosaic or Montage? Mosaic and there are different ones in GMIC doesn't have any of those settings. Montage does however. I always end up with the montage larger because it seems most of the pictures average out to the same size and then the size is multiplied by the amount of images - I usually have to make the completed montage smaller before posting.

You may find that 'scaling' should make your pictures in the montage all the same size whereas if you 'align' they will keep the approximate same size and fill in your montage according to the sizes.
This might help you more with your problem https://gmic.eu/reference/montage.html