Hi Marinos,
You wouldn’t need GB Pro to use custom icons.
To use custom icons in GB free version, here is a guide you may follow: https://www.youtube.com/watch?v=wdzFximLiQs
The difference in GB Pro version is you can store the icons for reuse through the Asset Library as shown in the video shared by Leo.
Lastly, there’s no additional icons in the Pro version. As mentioned, it only provides you the Asset Library in which you can save and store icons for easy reuse as opposed to copying and pasting the svg code all the time.
Hope this clarifies.