Thanks for purchase Logo plugin.
At Pro version of GS Logo slider, Logo height & width set to 200px at Logo Settings > Style Settings (3rd tab).
Also, you can set different Logo height & width.
All logos height & width MUST be same as set at settings.
Or you can delete the values (200) & keep it blank.
** You’ve to set at least one value for either height or width and all images have to be exactly that size.
If you set value, please resize your logos larger than 200px or larger than which you set, then reupload logos from computer, NOT from media.
Hope this will solve your problem.
If still face problem, let me know & send your site access (if needed) @ samdani1997 (at) gmail.com