Does the iphone 6 model Model A1586 (GSM) works in Brazil?

It will probably work but you may not be able to use 4G/LTE and you will have to return to Japan for warranty service. Only purchase the phone in Japan from an Apple Store and be sure to ask them if it is UNLOCKED, not SIM free not contract free. None of the Japanese cell carriers will unlock an iPhone.

It will depend on which bands your carrier actually uses and which are on the iPhone, it is normally multiple bands. No way to tell for sure until you try it, assuming you are going to use an approved iPhone cell carrier.

As far as I know they don't sell any unlocked iPhones in Japan so be sure to ask clearly and have it show on your receipt that it is unlocked. If it doesn't it will not work at all in Brazil. And again, if you get to Brazil and the screen fails 30 days later you will have to return to Japan to get it repaired.