1) Plan ahead. How many meals and snacks will you have for the duration? On a scale of 1 - 10, how likely are you to find gluten-free fare while you are gone? The lower the score, the more you need to plan and food you'll need to pack. Will you have easy access to a grocery store where you can pick up items you need?
2) Pack it in. Be sure to take enough food for the trip and for your first meal or snack at your destination so that you have enough time to solidify your plan on arrival.
3) Implement the plan. If you flew to your location and are renting a car, maybe you can stop at a supermarket for the things you'll need before you arrive at your final location. Maybe your mother-in-law will be making a run to the store and you can offer to ride along and then pick up a few things. For the big family meal at a restaurant, suggest a place that you've researched in advance and know has a gf menu. Chain family restaurants like Olive Garden have unthrilling gf menus that can be a definite back-up plan if you don't have a more interesting choice. If the restaurant has already been established, call in advance to discuss your options. Imagine what it will be like to get all of your meals and snacks handled, and if this part is especially challenging, pretend you are a secret agent on a mission to secure the top secret gf packages. Have fun with it! Since you've got a little food on hand that you packed, no need to freak out.
4) Enjoy your stay! Once you are secure in the knowledge that you will be able to eat safely (even if your food choices aren't thrilling, like in my video), then remember the purpose of the trip -- spending time with family, networking, or whatever. Be in the moment and have fun with it. Don't spend a bunch of time being bent out of shape that you have to do all these dietary shinanigans. Life's too short.
Safe travels, and let me know how it goes!