Hello.
I'm a journalist and media personality. I don't have GoPro, but many of my colleagues do. I have been looking at buying one. Most of my really professional work is in a real TV studio. A lot of my personal blogging and vlogging is done with my Samsung Galaxy tablet and phone. This is not what you are asking, but, I will assure you that my videos have been shown on TV news networks and the quality was great. I can provide you with some tips on how to kit a phone out, for less than $200, to make it a great video option.
Back to GoPro, Good investment, but, in my opinion, not very necessary for your project. If you plan to do more than this documentary, then it's a good idea. Frankly, I have two $1800+ digital SLR cameras and one very high end video camera that are collecting dust. This is my 4th Samsung. It fits in my pocket, a party purse, uses wi-fi and the videos are easily transferred to Vimeo, YouTube and a host of video editing software that is included in the Samsung purchase or free on Google Play.
If you'd like more detailed info on how to work the heck out of a cell phone video camera, check this out. This guy is widely followed by bloggers: http://www.jawavifilms.com/2015/01/20/the-only-2-cameras-bloggers-need-to-make-youtube-videos-hint-you-already-have-one-of-them/
I can also tell you about the accessories I use and where to get them cheaply.
I did several videos of my Granny before her death at age 91. I cherish this footage. It's been 7 years since her death. I'm SO grateful I decided to start that project when she was 84. I wouldn't steer you wrong with a memory so precious.
I'm here to help! I tutor on media subjects. I can help! Lucinda