After "info gnuplot", go to "Terminal_types::". One gets:
------------------------------------------------------------
4 Terminal types
****************

* Menu:

* complete_list_of_terminals::
------------------------------------------------------------

The next page is just:

------------------------------------------------------------
4.1 complete list of terminals
==============================

Gnuplot supports a large number of output formats.  These are selected
by choosing an appropriate terminal type, possibly with additional
modifying options.  See *note terminal::.

   This document may describe terminal types that are not available to
you because they were not configured or installed on your system.  To
see a list of terminals available on a particular gnuplot installation,
type 'set terminal' with no modifiers.

   Terminals marked 'legacy' are not built by default in recent gnuplot
versions and may not actually work.  @c <3 - all terminal stuff is
pulled from the .trm files

* Menu:

* Bugs::
------------------------------------------------------------

without the complete list of terminals, contrary to the manual
in HTML.

these two bugs (missing terminal types and broken navigation) were
due to the same cause, and I merged them, but these are really two
independent bugs, though they occur at the same place in the manual.
So I unmerged them a few hours ago.

This patch fixes the missing terminal types by providing a correct
relative pathname for the .trm files. Note that it is specific to
Debian, since the relative pathname depends on the location of the
build directory (a general fix would be more complex than this
one-line change).

Note: the menu with the "Bugs" item seen above is bug 989029 (this
menu is the cause of the broken navigation in "info", and I will
post a patch there).

The bug is still present and open upstream, and my patch still works.

The bug is still present and open upstream, and my patch is
still working.

Let's give a more detailed title.

I recall that the issue comes from the fact that for the build of
the Debian gnuplot binary packages, the build directory is not the
same as the source directory, while docs/doc2texi.el requires them
to be identical.

I had provided a simple patch for the build of the Debian package at

https://bugs.debian.org/cgi-bin/bugreport.cgi?att=1;bug=989028;filename=gnuplot-trm-path.patch;msg=26

which just does

Index: gnuplot-5.4.4+dfsg1/docs/doc2texi.el
===================================================================
--- gnuplot-5.4.4+dfsg1.orig/docs/doc2texi.el
+++ gnuplot-5.4.4+dfsg1/docs/doc2texi.el
@@ -116,7 +116,7 @@
 ;;; You may need to customize these variables:
 (defvar d2t-doc-file-name "gnuplot.doc"
   "Name of the gnuplot.doc file.")
-(defvar d2t-terminal-directory (expand-file-name "../term/")
+(defvar d2t-terminal-directory (expand-file-name "../../../term/")
   "Location of .trm files in gnuplot source tree.")

A patch to support various locations of the build directory would be
more complex.

The bug is still there.