Your Gyarados is 1619 > 1460.
The highest CP determines prestige/exp gain for the whole party.
To make it a bit more clear, let's say you're trying to do the bubble strat with CP 10's. You put a CP20 Krabby in an empty gym and then try to train with 5 CP10's and 1 CP 200. You get exp and prestige based on that CP200.
Friendly Gym Prestige Since Update
I need some help understanding how friendly gym prestige works since the update..
I've just approached a friendly gym with the following:
Pos.3: Vaporeon 1460CP
Pos.2: Snorlax 1887CP
Pos.3: Wigglytuff 1902CP
and I've lined up..
1st: Jolteon 1457CP
2nd: Gyarados 1619CP
I've beaten the Vaporeon with my Jolteon, then swapped out my Jolteon to start attacking the Snorlax with my Gyarados. I lost against the Snorlax then fleed.
Why did I only receive 324 prestige?
Previously if the CP was lower than the mon I was attacking I would receive 500+ prestige.
I'm level 29 so the CP adjustments did not change the 2 I was attacking.
Answers
This update changes the training gym mechanic a bit. Now it takes into account your highest CP among the six selected pokemons.
I think the formula is changed too, but I haven't seen anyone figure it out. We can just guess if you defeat a pokemon CP lower than your highest CP pokemon you will receive less than 500 prestige and vice versa.
Thanks for the explanation. I thought BubbleStrat was nerfed totally. Instead it is a bit boring to replace the high 6 CP Pokemon it will choose to train with 6 Pokemon of lower CP than the defender to get big Prestige up to 1000 per fight. Makes it slow and probably impossible to hold off attackers- as it used to be much faster to gain prestige just picking 1 attacker.