Your GPA is calculated across your most recent three years of tertiary study. As for what you need, nobody knows, but I would want 6+ to be properly competitive.
The GAMSAT has 3 sections: reading comprehension of literature (2 hours), 2 essays on a selection of prompts (1 hour), multiple choice chem/bio/physics questions (3 hours). It's a long exam, which makes it tough to focus all the way through but otherwise the questions themselves aren't extremely difficult.
The interview is an MMI, check online, there is a ton of information about them. You rotate through X stations for a few minutes at each.
I would say get your GPA right the first time because if you want to change it significantly then yeah, you need to go back to uni which isn't always an option.
Some bad marks from semester 1, year 1 won't hold you back much if you turn it around now. If you don't turn it around and keep scraping passes, then yeah it will affect your ability to apply etc.