GCL supports the CLtL1 specification. A function call is basically the same speed as a C function call, in fact identical to a C function call via a pointer. This is invaluable when trying to understand large systems. Has pioneered conservative Garbage Collection schemes. Also has a garbage collection scheme, for only recent allocations, based on native page fault handling.
People have huge chip verification suites which run for extended periods.
I have had computations which ran for weeks. There are no leaks. I have numerous users who claim their code runs two times under GCL, than in competitive lisps. Socket code is being added. A new Windows 95 port is in progress.