Don't go early. Don't even go on time. Nothing will start for a good 1 - 2 hours after the published start time.
Take a shawl to cover females' heads and plain white handkerchiefs to cover the males'.
The religious ceremony is very long and noone will expect you to be there for all of it. Part of it will be going on long before the bridal parties arrive. It is quite acceptable for guests to go and sit in for a while then come out again. Best to ask someone at the gurdwara when the key parts of the ceremony are likely to be happening and go up shortly before that time. In the meantime you can hang out in the communal eating hall 'the langar'.
Before entering the main relgious hall:
Remove shoes
Cover heads
Wash your hands
If you would like to pay your respects appropriately, have coins ready to donate to the box in front of the holy book - the Guru Granth Sahib. You walk to the front, kneel down, bow down and put in your donation.
Males/females do sit separately, but it is ok for very little boys to sit with their mothers.
Find somewhere comfortable to sit - the ceremony is very long and that is why most people will be slouched against the wall!
Take a pack of tissues in your handbag as at the end someone will unexpectedly give you some blessed food 'prashad', which is moist and very sticky. It is not good form to refuse or throw it away, although you can ask for a small amount.
At the end there will be lovely food in the langar, served on a thali (subdivided tray). Then you will probably be invited to a reception in another venue with yet more food and loads of dancing.
Men are offered alchohol at the reception, but it is not usually offered to women. However, probably noone would mind if you had some, as a 'visitor'.