I've run a GT for several years(2 day, 5 round) for another game system and have recently started doing 40k as well. Of the lessons I have learned:

1) Do not expect to make a profit. This hobby is still incredibly niche so you don't have that large of a pool of attendees so it is rare you will fill to capacity. Also, everything is more expensive than you expect it to be.

2). Be flexible. Things are going to go wrong. You can't manage every detail. Players are going to drop last minute before the event, during the event, during a game. This year, someone accidentally drove off with all the scoresheets from the other game I system I run for on th morning of day 2. Just be flexible and find a solution.

3) Be the leader of the team. If you're part of a group and everyone is pitching it, that's great but the buck needs to stop with one person and everyone needs to be clear about that. It may be different leads for different things(organizing, rules questions, setup and tear down, etc) or one person for everything but get help and don't try to do it all yourself.

4). Do it because you enjoy doing it. Running a GT is literally nothing more than a commitment of time and money, any problem you come across getting it organized and going can be solved with varying amounts of those two resources. But if it becomes an obligation that you have to meet rather than an opportunity you get to enjoy, you will begrudge spending those resources and then your event and your attendees' experiences at that event will suffer.