See the chapter 6.3 'The Free-Storage List and the Garbage Collector' in the LISP I Programmer's manual from March 1960.
http://bitsavers.org/pdf/mit/rle_lisp/LISP_I_Programmers_Man...
http://bitsavers.org/pdf/mit/rle_lisp/LISP_I_Programmers_Man...
The same site has the sources for plenty of other versions.
The manual also states that it's for "a version of Lisp being prepared for the IBM 709", implying the manual was written while the interpreter was still being developed, and all references I've found state that the IBM 709 version was the first practical implementation.
More on the very early history of Lisp here: http://jmc.stanford.edu/articles/lisp/lisp.pdf
So we have 60 years of GC research.