If you are new to the sport (climbing trees or rocks) I suggest postponing buying the Gri-gri. When I got mine I was very enthused about it but as you change your climbing stile it has been left in my back pocket in most of the climbs. When you \"graduate\" to SRT later on them the Gri-gri can be a nice tool to have in big long descends or while using the RADS system. There are two sizes of the Gri-gri that I have seen at REI. I have used mine in all my ropes from Sportline to fat Arborplex to KMIII ( it did not work well on this rope because it is too stiff to bend). I assume that the manufacturer recommend you to stay within the specified diameter based on their factory testing and years of experience but any rope could work \"in general\" with adult supervision (you).
Some ropes will go thru it easily, while others will not even fit in the unit so you do not use it with those. With the skinny ropes it runs a little bit too fast and you have to be careful because the Gri-gri does not lock up all the times and you can get a surprise drop. (speaking from experience here).
Warning:
The opinions expressed above are only my personal views and should not be considered as \"approved Training\". You should get Professional advise or training from a Certified person before attenting to use any new climbing gear. Serious Injury or worse can occur!