나는 내일 친구를 만날 것이에요 or 나는 내일 친구를 만날 거예요
I will meet a friend tomorrow.(very possible)
나는 내일 친구를 만나겠어요
I will meet a friend tomorrow.(quite certain)
갈 거요 is short form of 갈 거예요. means I will go (sometime in the future)
갈게요 means I will go immediately
가요 means I'm going
Quoted from https://hanguladay.com/tag/future-tense/
Probable Future distinguished by -(으)ㄹ 거예요
is the most common way to to refer future events in Korean. It can be used in all persons (1st, 2nd or 3rd person point of view). It can also be attached to all types of verbs, descriptive or processive.
is the more popular marker. Most refers -겠- as the marker that indicates future.
While this true, the future presumptive is a matter of fact statement of personal intention. It can only be used for first person statements and second person questions.
on the other hand can be distinguished by -(으)ㄹ게요.
It is always first person and the action is usually beneficial to the hearer. This marker can only be used in a processive verb or verb that takes a direct object.
Here are example to show you how the meaning differs using the 3 types of future:
읽겠어요 means I will read, am going to read. This is a strong statement of the speaker’s intention to read.
읽을 거에요 will read, am/is going to read. Statement in this form is less certain than the first.
읽을게요 I’ll read it/ I promise to read it. This makes the statement more immediate and gives a hint of promise or even assurance that the speaker is actually going to read.