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.

Example: 

<?
//some_file.sql  filesize = 2,048,000 bytes

$text_fp=fopen('some_file.sql','r');
$gz_fp=gzopen('some_file.sql.gz','wb9');
while(!feof($text_fp)){
    gzwrite($gz_fp,fread($text_fp,655360));
}
fclose($text_fp);
echo "gztell = ".gztell($gz_fp)."<BR>\n";
gzclose($gz_fp);
echo "filesize = ".filesize('some_file.sql.gz')."<BR>\n";
?>

Output:

gztell = 2048000
filesize = 249264

I will report this as a bug but post a note here for now