GZip compression does compressing "output" pages on the server while being sent to a client, that makes transaction faster. Client's browser must support such a feature (most modern browsers support it) and it decompress received page and shows it to the user.
I probably wont be "too wrong" if I compare this feature with Dialup connection to internet. Because 56k with v.90, v.92 and so on compression algorith does the same thing - fist compress requests and sents to the server then decompress received information and shows it to the user.
the only back-draw is that takes little more server recources to compress, and client recources to decompress, but if u have atleast "minimum" updated computer u wont be able even notice any slowing down.