CL-USER> (describe 'gensym)
COMMON-LISP:GENSYM
  [symbol]

GENSYM names a compiled function:
  Lambda-list: (&OPTIONAL (SB-IMPL::THING "G"))
  Declared type: (FUNCTION
                  (&OPTIONAL
                   (OR (VECTOR CHARACTER) (VECTOR NIL) BASE-STRING
                       UNSIGNED-BYTE))
                  (VALUES SYMBOL &OPTIONAL))
  Derived type: (FUNCTION
                 (&OPTIONAL
                  (OR (VECTOR CHARACTER) (VECTOR NIL) UNSIGNED-BYTE
                      BASE-STRING))
                 (VALUES SYMBOL &OPTIONAL))
  Documentation:
    Creates a new uninterned symbol whose name is a prefix string (defaults
       to "G"), followed by a decimal number. Thing, when supplied, will
       alter the prefix if it is a string, or be used for the decimal number
       if it is a number, of this symbol. The default value of the number is
       the current value of *gensym-counter* which is incremented each time
       it is used.
  Source file: SYS:SRC;CODE;SYMBOL.LISP