As a former athlete (and the father of a former college football player and two former college basketball players), it is my opinion that the gym can be an important part of a full program of fitness and strength. As said earlier on this thread -- the gym is where the equipment is, all in one place.
Weight lifting and other resistance training have changed a lot in the last few decades. Rapid sets keep the heart rate elevated build strength, endurance, and cardio-vascular fitness all at the same time. Complex resistance motions dominate training now and were unknown a few decades ago. And, yes, it is possible to get very strong without a gym, but strength does take resistance, hard work, and lots of reps no matter what method you use. A barbell, a hammer-strength machine, a log, or your own body weight can all serve the same purpose, but you do have to lift it to get the benefit.
Strength improvements made at the gym need to be put to use and translated into skilled motion. That is where getting into an environment of application (hunting woods, baseball field, boxing ring, etc.) and using the improved strength and fitness comes into play. Skill and strength are different training areas.
Every professional and college sports program has a weight-room routine. They do that for two reasons: first, because they have found there are limits to what can be efficiently done with body-weight exercises alone; second, because it pays off in measurable results -- measurable results: times and scores, not opinions.
Perhaps we can think of it this way: more strength and endurance means having more physical talent. Developing the talent into an applied skill is the next step in the process and both steps are necessary for me to be at my best (I am not a "natural" in either area).
My two cents worth and my humble opinion.
Joe