Every village needs an elder and Gisaku is the go-to guy for this one. He's incredibly old, for starters, which means he's survived a lot and lived to tell the tale. Rikichi may be pushing to fight, but it's Gisaku who suggests hiring samurai. Why? Because when he was much younger, he once saw samurai protect a village from a similar threat. He has the wisdom they need to find a solution to their dilemma, which is why they ultimately go to him when they're in trouble. He even suggests that they find "hungry samurai" to better convince them to fight for the peanuts they're paying him.
Gisaku also has a certain courage that comes from being old. He accepts that he's managed to live a long time despite the odds, and he finds peace in that equation. When the bandits come, he refuses to leave the mill where he lived, even though it's outside the zone of protection and the bandits end up burning him alive in the process.
That courage is another way Kurosawa has of blurring the lines between the peasants and the samurai who are supposedly their betters. This old man goes out on his own terms and nobody else's, just like the samurai wish they could do. That doesn't necessarily make him a samurai, but it does show how non-samurai can sometimes fulfill the tenets of their code. He has a grace in his death that his betters are chasing after, reminding his fellows that sometimes accepting the inevitable is the only way to find peace. If you want to live, you may need to suffer, he tells us, and in this case, he's quietly decided that his suffering is done.