mattb@Zed ~
$ ./gz.py
1000
1000
2000

mattb@Zed ~
$ cat gz.py
#!/usr/bin/env python

import gzip
import struct

data = '0123456789' * 100

f = gzip.open('foo.gz', 'wb')
f.write(data)
f.close()

f = open('foo.gz', 'rb')
f.seek(-4, 2)
print struct.unpack('<I', f.read())[0]
f.close()

f = gzip.open('foo.gz', 'ab')
f.write(data)
f.close()

f = open('foo.gz', 'rb')
f.seek(-4, 2)
print struct.unpack('<I', f.read())[0]
f.close()

f = gzip.open('foo.gz', 'rb')
print len(f.read())
f.close()