ET is 51 for the MKV GTI and Edition 30.
The 57 on the OP's alternative is the hub centre diameter that the wheel has to fit to.
ET 51 is for the OEM 7.5 inch wide alloys, but you can safely go down to ET45 on 7.5" wheels
If you go to 8" wide alloys then a wiser choice is anything between ET45 and ET48.
If you aren't lowered then you can just about scrape 8.5" wheels under the front at ET45 to ET42 without scrubbing under compression.