Is there a reason why you think pkgtools could be installed on a different distribution, but portage can't?

Anyway, the answer to "can pkgtools be made to work on Gentoo" is "no-one has ever tried".

You will never make any distribution easier to work on by bypassing or subverting the distribution native tools. The goal of "easy to work on" is served by keeping the system stable and predictable within the vetted parameters recognized by the support group and maintainers for that distribution.

You might violate that, I do daily, but do so with the knowledge that you make the system LESS stable and MORE complicated to support. If your skill set is currently inadequate for that task(as sounds likely), then you can take one of two paths:
1. avoid making your system unstable, or
2. plan on enjoying a learning curve dealing with the issues you will cause as you move forward.

I chose path 2., but that was over 30 years ago and I am still learning!

If you choose to go forward, I recommend considering backup and restore solutions first. That and keeping copious notes.

By "easier to work on", I mean that if I find an interesting application on the net, I can download and install it. Currently, I have to see if it is in my distribution's repository (of course, installing is very easy if it is in the repositories). If such downloads are in form of source.tar.gz, there should be an easy way to install them. I should be able to uninstall them as easily if I do not like the application. Checkinstall seems very good for Debian, rpm-based distributios and Slackware (apparently not for Gentoo & others). It appears that as Linux becomes more popular, application developers will provide runnable binaries, rather than distribution specific packages.