I don't think it automatically makes you more appealing - you could be an appealing candidate anyway without it.
However, it can show a commitment to a legal career at the same time.
It will depend on the firm as to whether your course gets funded if you have already started it of your own accord. Some firms pay back fees retrospectively whether you have started or completed the course. Others may pay a pro-rata amount for the time you have left on the course. Others will not pay back anything.
The interesting thing is that the GDL is no longer a requirement - its not like you have to do it anymore. Therefore to me it is a bit of a gamble just because it isn't necessary to complete it. For some firms, they won't put future trainees through the GDL, instead opting to just do an SQE prep course.