A big advantage of gcc is (other than having a free toolchain that runs on Windows, MacOSX and Linux) that there is no code size limitation of 32k (like the IAR kickstart edition), so you have 128k at your disposal without having to manually link smaller binaries (like they do with APP and LIB).
Personally I also dislike that the IAR projects are not compatible between versions so you find yourself with multiple versions installed, and each new release is 10x bigger than the last one.
Now, if you don’t need to change the LIB, basing your work on APP 2.5 sounds like the easiest way at the moment. But basing something on the closed-source LIB 2.5 is kind of a dead end. Sooner or later you will need to change something in the library and you will have to go back to 1.1. I really hope Ben or someone else will change this situation soon 
For a quick test, I would recommend using IAR if you already have it set up. I don’t think anyone has successfully recompiled the APP on gcc yet.