Hi David,
Contacts are used for frequently contacted call signs, email addresses, tactical call signs, etc. Pretty much the same as with many email clients and not related to gateways.
There is no need to download the Winlink Gateway Channels as WoAD can update this list by itself. From a packet session (or any other session except a telnet session) go into the session settings and navigate to Settings... Destination Address... RMS channel selection... From the lower menu there will be an option to "Update via internet". Selecting this will download the latest gateway list (assuming you have an active internet connection).
The list of gateways you are actually presented with will depend on the session type and your current location (which can be configured, using the top menu, from Other... Position report... You can enter a position manually or request one from GPS).
Hope that helps. All the best,
Andrew
VA7YAA