Okay, you unzipped the tar.gz file and now you have a .tar file, right? The .tar.gz is similar to a zip file, it contains a lot of files in one archive and then compresses it. In a zip file, this is done in one step. With a tar.gz file, this has two steps. First someone took many files and put them in one archive, which ends in .tar, then the tar file is compressed using gzip, and it becomes .tar.gz
Now, what you have done with 7zip is just undo the second part, the compression. You still need to get the files out of the .tar which is still an archive (just not compressed). All you need to do is untar it. I am sure 7zip can let you do that, too. So you probably just need to open it with 7zip again and ask it to unpack the tar file.
Next time, if you open a tar.gz file with any archiver program, and you all you see is a tar file inside, just double-click that tar file and it will give you the real contents of the archive.