Either do it the linear way or fun,more luck based way -

1) Linear - cook pizzas.I make 5 k per 2 hours just by cooking pizzas.
2) Fun way - get yourself a bee knife and search in forest areas for mushrooms,trash and acorns,it's much more fun,pretty much takes the same time to get 5k but a little bit more luck based.

I've only spent 38$ on Graal,been playing for 3 years and I've got like 520 k gralats by myself in less than 760 hours and I'm not even a no-lifer and this is my only MMO game.