GHANDI was legendary for his stubbornness.
Again, since this is a 'same-on' kanji, I don't really NEED to think of an ONyomi mnemonic, but this practically writes itself, plus this coffee is kicking in. . .
As babies, our original heads are stubborn. As we grow up, our heads tend to think more of others.
No exact English equivalent. がんばる combines two English expressions: 1) do your best! 2) good luck! You tell your friends 頑張れ！ before a big test or job interview.