Putting low CP POKéMON into a gym to 'train' it is one of the worst things you can do when you can just put a 1500~ CP one in and train with the same results. I was once training a gym here where I could use my ~ 1000 CP Tangela to defeat two slightly higher water POKéMON to get lots of rep, when another person from my team came in and dropped a ~ 700 Pidgeot in the slot I just opened up 'so he could train', basically cutting me off from training in the process. Another gym I'm in had a 10 CP Mankey and ~ 200 CP Ratticate that people put in there for 'training', and they're just two wasted slots. Just now I attempted to train a lv. 7 gym so I could put one of my POKéMON in, but it was near impossible because the low CP Mr. Mime that was in it wrecked all of my low CP stuff and did enough to my high CP stuff that they could not beat more than one other POKéMON, making training super inefficient.
If you want to train, put a high CP fire POKéMON in it and then train with a slightly weaker water POKéMON. Or a grass POKé and train with fire. Etc. Low CP POKéMON are wasted slots at best and can make training inefficient for others / cost lots of potions at worst. Yes, there are ways to make training easier by putting certain POKéMON in, but many people just do not know that, and in the end, attacking is still too easy when you can just bruteforce with 6 POKéMON at a time, only limited by the amount of potions you have on you.