GP not replenished upon level up

We get our HP, MP, TP replenished when we level up on a battle class, but how is it that when gathering, and we level up, our GP stay the same? I'm right now on my alt levelling botanist with leves, and it's so annoying that my GP don't get full after levelling up. I have to use Field Mastery almost on every leve I pop, and use cordials or stand around waiting for them to get full enough to use the next skill. I level up in between and still have to wait around when cordial is on cooldown D:
Why don't GP get replenished?

GP is so vitally important that it has its own unique system & it is unlikely to change. Or that's how the development team see it.