With UAG 3.5, I was able to import the UAG image to AWS and successfully executed the powershell script.
But while executing the powershell script, I commented out some lines as below link discusses...
I was having the same problem today for a critical-deployment upgrade of our UAG environment in aws and came across your post (and surprisingly little other documentation in the aws forums).
UAG is stored as a snapshot state not a regular disk image. if you check the vmware documenation Uploading the Unified Access Gateway Image with PowerShell you'll see that you need to use import-snapshot, not import-disk after you strip the vmdk out of the ova file. I fought with this for an hour this morning. Importing the image as a snapshot, not directly as an ami solved the problem immediately. Hope this helps! I know the post is stale, but hopefully this will help anyone who has this issue in the future!
Please check out this tutorial on Tech Zone for deploying the UAG. There is a section specifically on deploying UAG on AWS.