First things first—there’s no tangible advantage (or indeed disadvantage) to having the GDL when you apply for a training contract. Firms frequently pay for and organise GDL places for their future trainees, so it’s not really a factor in deciding whether or not to offer someone a training contract.
However, self-funding the GDL before applying for training contracts might put you at an advantage if you have not attained as high a grade as you’d like in your undergraduate degree. Putting a lot of work into your academics at GDL level could give you a better grade to offset any lower ones. While it won’t “cancel out” a 2.2 completely, it can certainly demonstrate your ability to work academically and score highly.
Another reason why doing the GDL could put you on a better footing in applying for training contracts is that you’ll have an extra year (or even two years) to build on your work experience, commercial awareness and general law CV.
The vast majority of GDL students have not done law prior to converting. So it might be that you decided to pursue a law career later in your undergraduate degree, after a lot of the work experience opportunities had passed. If this is the case, you can utilise the GDL year (or two years) to apply for vacation schemes, read as much around the commercial world as possible and engage in some volunteering and pro bono work. The GDL itself won’t make it easier to get a training contract—but the latter three things will.
You can also use your GDL year to research training contract opportunities, looking at the different areas of practice as you learn about them, and figuring out which firms are most appealing to you. This extra time will give you the insight you need to target and tailor your applications.
In short, the more time you spend studying the law in any capacity, the more time you have to embark on the career-building endeavours that will get you a training contract. So while we can’t put an exact figure on how hard it is to get a training contract after doing the GDL, we can say that putting in the effort now will pay off later.