The GAPAN test has nothing specifically to do with your prospects as an airline pilot, but it is a good indicator of your realistic chances of making it through your CPL/IR training. Aptitude assessment is not a matter of saying 'this guy's almost an airline pilot already'! It's far more about your ability to absorb information and put your learning to good effect within a very short timescale, as well as basic motor skills, spatial awareness etc.
There are many, many candidates who do not get high scores. Equally, there are many pilot trainees who eventually achieve a CPL/IR without a great deal of aptitude - and if I throw enough money and hours at almost anyone, they will pass eventually. However, they may very well not make good airline pilots. One of the advantages of having that GAPAN assessment is that, along with your history of first-time passes in minimum hours, you will be able to reassure your prospective employer that they will be able to train you on their aircraft at minimum cost and in minimum time. That, believe me, is important!
Scroggs