There isn't a gap anywhere between Adam and Abraham.  There is a complete list of people saying when one person was born, the previous person was a certain age.  Even if there were one or more skipped generations, that doesn't change the fact that the Father was  the specified age when the son (or grandson) was born.  The genealogies lead to a period when dates are known and can be continued.  It isn't that it was 6,000 years between Adam and Noah.  It has been about 6,000 years since Adam.  The Bible is very specific.

It is very interesting to take the genealogies in a spreadsheet and see how they overlap and how the years match up.  It helps you see how much time has gone by.