The Lispiness that they took advantage of was metaprogramming via s-expressions, an interactive REPL, and dynamic updating of functions during runtime (which would require some form of GC in function memory footprint, but wouldn't be a drag on normal gameplay timing).
GOAL's ability both to execute code at the listener and to replace existing code in the game at run time introduced the problem of memory usage, and more specifically, garbage collection. As new code was compiled, older code (and other memory used by the compiler) was orphaned, eventually causing the PC to run low on free memory. A slow garbage collection process would automatically occur when available memory became sufficiently low, and the compiler would be unresponsive until the process had completed, sometimes taking as long as 15 minutes.