I’d be a bit surprised if PTGui would reassemble the images. Most image stitching software I’ve used (it has been a few years) required an overlap between images and a focal length of the lens in order to do the stitching. GIMP is free and will do the job. Affinity Photo is often touted as an alternative to Photoshop. It was on sale a month or two ago for $25. I’d expect some ramp up time on either app.