Sometimes applications in the Software Center are outdated and several years old. For instance the version of QtGain is several years old.
Who should I contact to report this?
You will actually want file a "needs packaging" bug and contact the package maintainer, not the developer of the software (although sometimes they are the same person).
The 'needs packaging' bug will need to be filed on launchpad against Ubuntu
To find the package maintainer, you can either search for the package on http://packages.ubuntu.com/ or from the command line with
apt-cache show <packagename>
You could then send the maintainer an email with the bug number.
The developer of the software. For very old projects, they may not be interested in updating the application, in that case you could look for an alternative (or fix it yourself :)
In the case of QtGain, though, it seems to have been updated only a few months ago, and there is some sort of support thread on the QtGain page at QtApps.org. The developer is known as Vegeta, and the only way to contact him seems to be through the afore-mentioned thread.
Note that the versions of applications in the Software Center (or the repo) are often not the very latest ones; in case of large gaps between the latest version and the in-repo version, you should contact the package's maintainer, or simply use the latest version and bypass the Software Center / package manager entirely (for that software).