I ran this race with some friends from this forum last year.
The weather last year was cool/cold. The conditions suited me just fine. I think warm weather would be more unusual that far north. It'll most likely be colder than what you train in leading up to the event.
Food wise there are a few Italian restaurants in the town where you can get lunch. Tromso isn't large and a quick bit of googling should be able to get you websites, or phone numbers to check that they cater for GF diets. There's no language barrier, they all speak perfect English.
Pre-race fuelling and getting it right is possibly more a case of trail and error. A group of us had an early pasta lunch and most felt fine that night. On the other hand I had my stomach churning for the opening few miles. That may have just been a coincidence though.
After the event last year there is a small event area at the finish with beers and some other offerings. There isn't a huge amount of things open at that time of night, but a few hotel bars and bars will be open.
On the Sunday there is a prize giving event if you want to attend. Last year a few of us went for a walk and took the cable car to the viewing area at the mountain overlooking the town. There's a nice little restaurant/bar up there is you want some drink/food and a view.
On the eating and drinking thing the one word of caution I can give is be careful not to over estimate how much you need. For my first marathon I think I was eating for a family of six, as I though that was what I needed to fuel my runs!
My own personal opinion is that if you're diet is reasonably healthy then you don't really need to adjust it. For long training runs like your 18 miles you do need to use the time between now and Paris to find out what works for you as a pre-run meal. For many that is porridge. I actually seem to run well off crunchy nut cornflakes. Ultimately it's trial and error until you find something that works for you. The same approach for race fuelling. Try a few different brands of gels, or shotbloks, or other foods you can eat on the run and see what sits well in your stomach while you run.
Another thing is don't worry about that 18 mile slog, Been there, done that. In fact just the other weekend I had one of those training runs and I could have happily quit at any mile along the way. A real slog from start to finish. It's just a thing of doing a long marathon training schedule. Sometimes no matter what you have a day where it just feels like hell from start to finish. Those runs are great for building up the mental toughness. Other times you'll get a surprise feeling good run and it'll boost your confidence.
As someone new to the distance I'd expect you're first few long runs like that 18 to be a slog. Your body isn't used to that distance yet and you'll still be working out what is best to eat pre-run and during the run.
Andy - That's still some very impressive mileage you've racked up over the last few years.
Pancake - Don't worry about some of us having more experience. We all started off somewhere and some of us with far more idiotic plans than others!
The marathon is a daunting distance, but you'll get to the finish. There are few more memorable moments than completing your first marathon and the feeling of knowing that you did something that even on the start line you doubted you could do.
It's also worth remembering that while it's good to read about other peoples training on here, just because some are doing more miles, or going faster, doesn't mean you aren't doing enough or can't talk about your training. Everyone is at different levels, different experience and many started exactly where you are now.
The thread has been great over the years at supporting people in achieving their goal, be that to finish or going for a specific time. You've picked a great race for your first marathon.