I was so excited to see the deal on appsumo, as I love optimizing WordPress to make it load faster also offer similar service to clients, so a new tool to play with. Here is a review and comparison with shortpixel after playing with it for an hour.

1) WP Plugin -
Shortpixel's WP plugin is more mature. Gumlet didn't load my featured images from their CDN. Their wp plugin needs more work. (remember Shortpixel was in beta for more than 6 months before AS offering )

2) File Size-
Both SP and Gumlet are more and less equal. The file size of default compression in Gumlet is almost equal to Shortpixel's Lossless compression with a less than 10kb difference.

Here are the details-
Original Image- 684kb
Gumlet-
Image quality set 80 (default value) - 371KB
Image quality set 70- 275KB
Image quality set 60- 240KB
Shortpixel-
Lossless- 379KB
Glossy- 325KB
Lossy-251KB

3)On the fly compression speed-
Gumlet shines in this. In shortpixel, the first request to a CDN pop results in 307 redirects to the original image, the uncompressed image loads, and from the second hit, it loads the compressed image from SP's CDN. The same isn't case for Gumlet.

4) Pops-
On paper, Gumlet has more number of pops rather than shortpixel, as it uses AWS CloudFront vs. shortpixel's stackpath. Cloufront is faster than stackpath too.

5) Cache-control-
Gumlet lets you control cache timing, which shortpixel doesn't.

6) Cache purge-
You can purge the cache from Gumlet's dashboard; for Shortpixel, you need to ask the support.

Regarding image quality I have uploaded all of them here you can check them here- https://www.notion.so/wpfy/SPAI-VS-Gunmlet-453a0081574e4b43b7dbd89cadf3f757

Verdict- If you don't have shortpixel or running out of its credit limit, then you should get it. One star less for half baked wp plugin.

It is my first review on AS, paradon me if I missed something.