The g:param type specifier represents the fundamental "GParam" type.

(g:symbol-for-gtype "GParam")
=> GOBJECT:PARAM
=> T
(g:param-spec-int "name" "nick" "blurb" 0 100 50 :readable)
=> #.(SB-SYS:INT-SAP #X5DEC9A4A41C0)
(g:type-from-instance *)
=> #<GTYPE :name "GParamInt" :id 103270782157856>
(g:symbol-for-gtype (g:type-fundamental *))
=> GOBJECT:PARAM
=> T