Gun has a unlock system to get you to the final boss fight. In all honesty, their achievement system is asinine, requiring 4 full playthroughs to get the achievements, if you beat the game on normal, you have to go back and play the whole thing again on easy, or you can just do the above mentioned jump to the end. The insane difficulty final boss fight is still extremely difficult. And to get 1000/1000 you have to do a 100% completion playthrough once first, which takes 8-10 hours.
It is definitely a 7/10 if you go and play it through 4 times. It's probably a 4/10 if you do the jump. The final boss fight, again, is REALLY difficult on insane, and took me dozens of tries to get through it.