So I actually readed (some% )slackbook and googled but I'm still not sure do I need to keep .tgz's somewhere so I will be able to uninstall package from 'pkgtool'.

Or will slackware keep track of installation by itself so when using 'installpkg some-package.tgz' I can remove that package and afterwards still be able to uninstall it...?

You don't need to keep the package to uninstall it - if you have installed with the stock "installpkg". In /var/log/packages all installed packages are being tracked and this information is enough for "removepkg" to do its job.

There's no need to keep the tgz. Every package you have is listed in /var/log/packages. Each one is a file that has a list of the files, directories, etc. that will be removed if you remove the package.

Anything else is usually in $HOME/.SomeHiddenDirectory.

Every package built with a correctly written SlackBuild script and installed with installpkg will be listed in /var/log/packages and therefore taken into account in the Slackware package management system. For those, you do not need to keep the .tgz packages. If you, however, build/install something via simple (not recommended):

or executing a binary package (eg. nvidia binary driver, VirtualBox, etc), you should keep the installation directory or executable binary file to make it easier for you to uninstall it in the future. They are not included in the slackware package management system.