Gunzip doesn't recognize a .mat.gz file as a GZIP

Hi! I've received a package of zipped datas. Since it was impossible to access them I've used 7zip to access the subfolders. 7zip couldn't unzip the gzipped single files, so I've opened MatLab and tried on every gzip file to use gunzip:

gunzip 15*eeg1.mat.gz
gunzip 20*eeg2.mat.gz
gunzip 20*eeg3.mat.gz
gunzip 20*eeg4.mat.gz
gunzip 20*eeg5.mat.gz

Nothing special... but with some files in some folders it has worked and instead with other folders it returned to me these errors:

Error using gunzip>gunzipwrite (line 184)
File "Rat20-1.zip" is not in GZIP format.

Error in gunzip>gunzipEntries (line 122)
  names{k} = gunzipwrite(entries(k).file, outputDir, baseName, streamCopier);

Error in gunzip (line 64)
   names = gunzipEntries(entries, outputDir);
But the files still have the extention .mat.gz... What is that? What can I do?

[K E]

I get the "not in GZIP format" error on one machine, but not on another! Would love a solution.