The one caution about Gigapixel AI is that it seems to be somewhat image dependent on how good are the results. It is good to have options!
I must second that. Overall, Gigapixel does a better job, but it seems to accel with lower res images in general. With higher res images you are taking up even larger, I have found PhotoZoom is faster and seems to do an almost equal job in far less time.
On a recent job, I needed to enlarge a horizontal image (I think originally captured with a 5D Mark II or III) about 1200% to then be cropped vertically and printed as a backdrop 10'x16'. I experimented with a small section and used Super Res, Gigapixel and PhotoZoom. The two latter programs did better than Super Res with almost the same results in the end. I think I used Gigapixel for the final, but had to do two passes of 600%.