You can do it but it will involve two separate searches. One to search for the GZ files and a second to search inside them.
A simple search for:
*.gz with a Modified date: of
Today -24 hrs will find the target GZ files, e.g.
Then to search inside those found files select the menu option File->Search Result List. Make sure that the GZIP extension is switched on in the Compressed Files tab and you can then search just those GZ files found in the first step:
Using just the command line
If you want to do the same thing via an automated process you'll need to add an extra step to export the list of GZ files found to a text file. There's a specific custom format for exporting just the file names called
"C:\program files\mythicsoft\filelocator pro\filelocatorpro.exe" "E:\SearchForGZ.srf" -o "e:\files.txt" -ofxslt "C:\Program Files\Mythicsoft\FileLocator Pro\Sample Transforms\fullname_only.xsl"
Then using the File List functionality specify the generated text file in the Look In field to target the GZ files to search, e.g. as seen from the UI:
Note: The '=' sign in front of the text file is very important. Without it the text file would be searched as a normal file, with the '=' sign the text file is used to actually LOAD the Look In field.