"So the Gla is maybe the key factor ."
"If we have more bad eicosanoids we lower the dose of gla and if we have too many good eicosanoids we increase the gla."
Yes, but this is not a hard and fast rule that you should only change GLA. You could also increase EPA for too many bad, or decrease EPA for too many good.
Whether to address an eicosanoid imbalance by changing the dose of EPA or GLA, or both, will depend on the individual's situation, such as how much EPA and GLA they are now getting, their particular symptoms, etc., but in general, yes, you're understanding correctly with this statement you've made about about good/bad/eicosanoids and GLA.
You're welcome. I'm happy to help!