Criminology is obviously a very different course to law so I think you need to consider how well you can sustain your motivation for doing it. The GDL is competitive and difficult so they look to take graduates with good degrees. Criminology itself will be absolutely fine for applying to the conversion, but if you're not interested in the subject then that might affect your results and your eligibility.
I guess you've got a deadline to decide by, but spend as long as you can researching the course and thinking about whether it's what you really want. The worst thing would be to accept it, find you don't like it, get a bad 2:2 and discount yourself from ever applying to the GDL.
There's nothing wrong with taking a gap year and reapplying to law at different unis, and the gap year could give you a good opportunity to get some work experience.