Pokemon-go – Does it help the team’s gym if I train there, but lose

When you battle against a gym that belongs to your team (training), if you win, you get an XP bonus for yourself, and a Prestige (reputation) bonus for the gym. Enough of the latter and the gym levels up. Higher level gyms can hold more Pokémon (one per trainer), which makes it harder for opposing teams to take over the gym. If you lose against your gym, no bonus for either you or the gym. The Pokémon you initially put in the gym stays there until it's bumped out by the gym losing enough Prestige to drop one or more levels (the lowest CP Pokémon drop off the lower end of the ladder, essentially). The gym loses Prestige by having opposing teams win against it.

If your gym Pokemon is returned to you as a result of the gym losing enough Prestige, your returned Pokemon is returned fainted, with no health, requiring healing via a Revive and Potions before another battle.

In your example, I don't think either of your Pokémon (Charmander left at gym to train, or the Pinsir you bring in to challenge) gain anything (it'd make sense if they did, but I haven't seen it).

Pokemon don't gain CP at a gym. Successfully training at a gym raises only that gym's "prestige," and consequentially, the gym's level.

The only way to increase the CP of your Pokemon is by using Stardust to power them up or evolve them.

The purpose of a gym is to put a Pokemon on it to defend it which can earn you 10 gold coins each (max 100) per 21 hours. You can collect them in the shop at the right top. This can be done straight after or you can wait.