The game has different layers. Personally I just try and catch pokemon. 'Winning' would be getting all 150.
I change the names in my replacements to something like "The King".
For a level 10 pidgey that's a lot of confidence.
I do this for a few reasons: 1) I can. 2) It's fun and keeping gyms is ridiculously hard. 3) It gives new players a chance to unseat my gym and get their daily bonus. 4) Powering up gyms is way under powered at this point. 5) This also gives other people on my team to power up the gym and reach their own goal if they like.