This is based on how I view and use the two.
GMB is a funny way of saying don't get married or be careful who you marry.
MGTOW is about avoiding women.
There's a fundamental difference in the two philosophies. GMB is about the institution of marriage. MGTOW is about women specifically. GMB isn't about avoiding women, it's about avoiding alimony and losing half. Divorces tend to favor the woman or the person who brought less to the relationship (typically the woman, because statistics show that men make more). Extremely wealthy men shouldn't get married to the average woman who isn't on their level financially. It's too much risk involved.
MGTOW is about women. We all know what Game is right. Only men with Game can play. Those with no Game have to either step it up or quit. MGTOW is a result of men with no Game quitting the Game. Dudes who lack savoir faire, panache, gumption are turning their backs on the Game AND develop the rationality that it's women's fault for not liking their corny asses.
See the difference...
GMB: "She got everything and he got to pay $100,000 a month for alimony. The SYSTEM ain't shyt

"
MGTOW: "Women don't like me, therefore WOMEN ain't shyt

"