Can you run injury-free and not do GSM? Sure, but why risk it? Said another way, think of GSM work as an insurance policy against injury.
The problem with GSM work: Most people find it less enjoyable than running. You're not moving from point A to point B, but rather doing work at the gym or at home before and after your run. I get that; I really do. But trust me on this—if you do GSM religiously for two or three weeks, you're going to feel different in your hips, and your posture will improve. You'll feel stronger and more powerful when you walk. Then, when you resume your run training, you'll be ready to handle all of the running that you want to do safely.
Make Sure You're Bored Before You Resume Running
There's an interesting phenomenon that occurs among the best runners in the world. They don't exactly know when they're ready to come back from their offseason break, so they simply wait until they're bored. Their breaks could be as short as 10 days, or as long as a month. For most elite athletes, the offseason break consists of one week of nothing, one to two weeks of cross-training and some light GSM (since they will usually complete a great deal of ancillary work once their training resumes), and then it's time to get back to running.
My experience reveals that recreational athletes wait until they're bored for an hour, and then decide to go for an hour-long run. Better to be bored for several days, then start with an easy 30-minute jog. Even then you might say, "I still want some more time off." Remember, the body needs to rest and recover. If you don't give your body a break now, you run the risk of becoming overly fatigued during your serious training, or getting injured because your bones, tendons, ligaments and muscles haven't recovered from the last race—especially if that last race was a marathon.
So be smart about your offseason break. Make sure to start a daily GSM routine, and then wait until you're really bored before you resume serious training.race.