Not really a question but a discussion

Then the CP (Combat Power) needs to be changed into AP: Attack Power; otherwise it is OK.

You can already look at the Defence stat of any mon (e.g. choose the "Pokemon list advanced") from the menu on this page, then click the header to sort by the Defence stat.

But in practise this number isn't terribly useful on its own, because effective defence strength is a combination of both the def stat (which determines how much damage each enemy attack does to you) and by stamina (=HP) which determines how much damage you can take. It could be useful to combine both of these into an overall "effective health" stat, e.g. effective health=(DEF/200)*HP. for example, Cloyster has only 100 stamina (i.e. little HP), but very high DEF, so the effective health is quite good: 162 'effective health' by this measure. By comparison, Hitmonlee, who has only slightly lower overall CP than Cloyster and also has 100 stamina, has a lower DEF stat, so only has 106 'effective health'. Ignoring type advantages, the difference between those numbers would be fairly indicative of how much faster Hitmonlee would go down against an enemy.

CP should favor attack. The other two stats accomplish the same thing (staying alive longer), so if they were equally weighted with attack it would boost tanky pokemon even more, and if that's what gym ranking was determined by, then gyms would be filled with sky-high-DP Chanseys and Snorlaxes that nobody wants to waste the time to kill.

To get to the root of the problem you would have to change the gym placement system more radically than this. And the problem, just to be on the same page, is not only the omnipresence of high CP poor defenders like Gyarados and Rhydon, but also overall lack of diversity.
The problem really boils down to the fact that the most important aspect when choosing defenders is a single (arbitrary) metric. Changing the metric will result in different Pokémon dominating gyms but not contribute to diversity.
Seeing only Poliwrath, Slobro and Chanseys would be only marginally better in my opinion.
One possibility would be to implement a prestige reward placement system where the more prestige you've added to the gym the higher your Pokémon is placed. This of course would have it's own shortcomings, i.e. bubblestrat

Just get rid of CP as gym placement criteria... use prestiege gained by the player or pokemon level, CP is a really bad meassure of how good a pokemon is both attacking and defending. This would free us so we can put the pokemon WE think is best for any given situation, not just the one with the highest CP