When I am importing say 300+ lines, I wanted to use the Group ID function. So what is Group Id, is it the same as Group by .......when we write SQL script?

You can group the data you are importing into Oracle GL using group ids.
In the GL Interface table there is a column called GROUP ID. You can put a number in there in order to identify which group the record belong to. Then when launching the journal import program you can specify the GROUP ID. System will import only the records belonging to this GROUP ID. This feature is helpful when different group of people in Finance department are responsible for importing part of data.
Here is an example. Lets say this is a sample data in GL INTERFACE Table.
(Journal, Account, Debit, Credit, Group Id)
JOURNAL1, ACCOUNT1, 100, 0, 1
JOURNAL1, ACCOUNT2, 0, 100, 1
JOURNAL2, ACCOUNT1, 10, 0, 2
JOURNAL2, ACCOUNT2, 0, 10, 2
Group 1 can run the import for GROUP ID = 1. Only Journal 1 will be imported.
Group 2 can run the import for GROUP ID = 2. Only Journal 1 will be imported.