Extract timestamp from a gzip file

Don't use gzip, use zcat instead which doesn't expect an extension. You can do the whole thing in one go. Just try to zcat the file and, if that fails because it isn't compressed, cat it instead:

for f in *; do 
    ( zcat "$f" || cat "$f" ) > temp && 
    mv temp "$f".ext && 
    rm "$f" 
done

The script above will first try to zcat the file into temp and, if that fails (if the file isn't in gzip format), it will just cat it. This is run in a subshell to capture the output of whichever command runs and redirect it to a temp file (temp). Then, the temp is renamed to the original file name plus an extension (.ext in this example) and the original is deleted.

Split and join of the big file works, but it is impossible to decompress pieces of the compressed file, because essential informations are distributed through the whole dataset. Another way; split the uncompressed file and compress the single parts. Now you can decompress each pieces. But why? You have to merge all decompressed parts before further processing.