what is different 겠다&ㄹ/을 것이다

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겠다, compare with ㄹ 것이다, has more meanings.
ㄹ 것이다 is just future tense.
While 겠다, has different meanings depening on who/what is the subject of sentence.
If the subject is "I", then 겠다 means hard intention to do something.
(제가) 열심히 공부하겠습니다 - I will study hard!
If the subject is "It, he or she", has the meaning of your guess or that something is expected.
그 사람은 벌써 도착했겠다 - Maybe he has already arrived (guess)
와, 너무 맛있겠다! - Wow, It looks/I think it's delicious!
내일 날씨가 춥겠습니다 - Tomorrow's weather is going to be cold.(expectation)
And some other meanings. To see others just find in internet

But in the meaning of future tense, ㄹ 것이다 is just simple fact that something (maybe) will occur in the future, while 겠다 when the subject is "I" means intention.
So, 내가 할 거예요 - Just fact that you will do something.
내가 하겠다 - You will do something, but it's intention.