Outdoor sports vs Gym training is not actually a one against the other. Many a times they are complementary to each other. Having said that, each has its own advantages which might be difficult to achieve in the other form.
Advantages of outdoor sports:
- Most of the outdoor sports actively target the whole of your body than specific parts of your body.
- Outdoor sports which do not require specific equipment (eg football, volleyball, running) are way more easier to participate in than a gym workout (which might require weights in case of weight training and machines in case of machine training).
- Being close to the nature might keep you more motivated as compared to a closed environment of a gym (this is a personal choice though).
Advantages of gym workouts:
- You can train specific muscles and improve the strength of those specific parts. This is especially beneficial for rehab scenarios.
- In case you are a beginner who is not too confident with your physical abilities, building strength is much easier in a gym where you have a much controlled setting. Machines for example help you develop a good form while doing certain movements.
- Weather proof. You need not worry about the weather being too hot or too cold outside. Unfortunately this is a reality that can prevent you from outdoor sports.
My personal view:
I do a bit of calisthenics (which doesn't require a gym and could be outdoors as well). I hike a lot which would in a way act as an outdoors sport and used to be a volleyball player in my younger days.
What I've realised is, my workouts in gym (calisthenics in this case) actually benefits my outdoor pursuits. And overall, both in different ways help you to stay fit and have an active lifestyle.
Based on the comment, how does one complement the other (some of the scenarios):
- Explosiveness. With outdoor activities like sprinting, you need explosive abilities. These can be trained in a gym.
- Muscle training for climbing - You can train for lat strength, biceps et al in gym which helps you in the actual outdoor sport.
- Running/hiking - Helps you to build overall stamina and endurance. This in turn feeds back to your abilities to improve your gym capacities.
- Swimming - this helps you with your breathing which is one key component for any physical activity (including gym activities).
Some resources that can help you see how different activities benefit from gym activities: