As long as PowerAmp can read the tags, they should be seen there. Tags are tags. If MB creates them, and you sync or copy the files to PowerAmp, then the tags are there. PowerAmp has to support the replay gain tags. And I'd be surprised if it doesn't. MB doesn't selectively decide which tags to send. They're all embedded in the file and they all get sent.
Check the files using MB's Tag Inspector (open the Tag Editor and click on the Tag Inspector tab) to check if the tags are present. If they are then I suggest you seek out a PowerAmp forum to find out if they're supported and if so, why they don't seem to appear.