ok so this function returns the gz file pointer as the uncompressed data byte length so if you are trying to put something in to specific size gzip files it won't work.
//some_file.sql filesize = 2,048,000 bytes
echo "gztell = ".gztell($gz_fp)."<BR>\n";
echo "filesize = ".filesize('some_file.sql.gz')."<BR>\n";
gztell = 2048000
filesize = 249264
I will report this as a bug but post a note here for now