LinuxTag is an annual event taking place in Berlin, Germany. Usually, about 10000 people visit this mixture of conference and expo.
For next year, please have a look at the CfP and submit GNOME-related stuff, e.g. news from the GNOME community, or talks about new code.
If registered early enough, Free Software projects (such as GNOME) can get a free booth including furniture. Booths are quite big with a lot of space for posters and flyers, so make sure you have those things. Also, have enough stuff to give away or sell, i.e. stickers, buttons or t-shirts. People want that type of stuff.
LinuxTag usually starts on Wednesday and it's possible to set up the booth on Tuesday. It's probably good to be able to start off fresh and shiny on Wednesday so try to build the booth on Tuesday. It also allows you to buy missing stuff on Wednesday.
Details for the events can be found here: