Only real advantages to the GSL are quite subjective, I bought it mainly because I do have a fair number of 23mm devices, much like the GTII the knurling is larger, prevents it from going in the Phantus, Kato, or any thing similar that maxes @23, Gizmo 2 is fine.
Airflow is same or minimally different, flavor seems to be same, I haven't done a side by side, if it is different, it isn't by much. Wicking is the same, with the exception it has room for more of it. The additional bulk between the 22mm, and the 23mm is quite noticeable to me. The additional juice capacity might be appealing to some, for me it makes no difference, doesn't bother me to refill when it is as simple as it is.
If you already have a GSII I don't see any reason to get the GSL, other then just wanting one. The original GS was one of the most annoying atomizers I own, it was very temperamental, mine would wick perfectly tank, after tank, then for no obvious reason it would dump a tank out the air holes. I'll admit though it seems it was user error, I still use mine occasionally and have no issues with it now. I believe the key differences in the build are larger coils, more wick. If the GSII is underwicked it will dump the tank, though it is not as apparent due to the reserve capacity of the well.
And yes, they do like the juice!