The rarity of the 2nd generation Galaxy Fighter is determined by the weapons you use to train it, and unlike forging, you will be able to use a combination of both ranged and melee weapons as training ingredients. Using higher tiers weapon and OG weapons for training will significantly increase the chances of receiving a rare 2nd generation.
The Generation 1 Galaxy Fighter used in the training process will also have an effect on the rarity of the Gen 2 fighter, the stronger and more rare the Gen 1 fighter is, the stronger and more rare the output will be:
2nd generation fighters are special for the following reasons:
On average stronger than Gen 1 fighters
Ability to earn Gold Keys
Participate in the exclusive weekly tournaments of $ETH/$GCOIN prize