Based on http://sources.debian.net/patches/sgt-puzzles/20160429.b31155b-1/202_online-help.diff/
Simply opens HTML help files with xdg-open

Author: Ben Hutchings <ben@decadent.org.uk>
Description: Add HTML-based online help

This works along the same lines as the Windows implementation,
though we have to try a bit harder to find a help browser.

--- a/gtk.c	2016-12-08 07:53:06.812409184 -0500
+++ b/gtk.c	2016-12-08 07:58:06.426415886 -0500
@@ -2,6 +2,10 @@
  * gtk.c: GTK front end for my puzzle collection.
  */
 
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 1 /* for PATH_MAX */
+#endif
+
 #include <stdio.h>
 #include <assert.h>
 #include <stdlib.h>
@@ -10,6 +14,9 @@
 #include <string.h>
 #include <errno.h>
 #include <math.h>
+#include <limits.h>
+#include <unistd.h>
+#include <locale.h>
 
 #include <sys/time.h>
 #include <sys/resource.h>
@@ -2270,6 +2277,37 @@
     resize_fe(fe);
 }
 
+static void show_help(frontend *fe, const char *topic)
+{
+    char path[PATH_MAX + 1];
+
+    sprintf(path, "%s/%s.html", HTMLDIR, topic);
+    if (access(path, R_OK) != 0) {
+        error_box(fe->window, "Help file could not be found");
+        return;
+    }
+
+    const char *argv[3];
+
+    argv[0] = "xdg-open";
+    argv[1] = path;
+    argv[2] = NULL;
+    if (! g_spawn_async(NULL, (char **)argv, NULL,
+                      G_SPAWN_SEARCH_PATH,
+                      NULL, NULL, NULL, NULL))
+        error_box(fe->window, "Failed to open help file");
+}
+
+static void menu_help_contents_event(GtkMenuItem *menuitem, gpointer data)
+{
+    show_help((frontend *)data, "index");
+}
+
+static void menu_help_specific_event(GtkMenuItem *menuitem, gpointer data)
+{
+    show_help((frontend *)data, thegame.htmlhelp_topic);
+}
+
 static void menu_about_event(GtkMenuItem *menuitem, gpointer data)
 {
     frontend *fe = (frontend *)data;
@@ -2590,6 +2628,25 @@
     menu = gtk_menu_new();
     gtk_menu_item_set_submenu(GTK_MENU_ITEM(menuitem), menu);
 
+    menuitem = gtk_menu_item_new_with_label("Contents");
+    gtk_container_add(GTK_CONTAINER(menu), menuitem);
+    g_signal_connect(G_OBJECT(menuitem), "activate",
+                     G_CALLBACK(menu_help_contents_event), fe);
+    gtk_widget_show(menuitem);
+
+    if (thegame.htmlhelp_topic) {
+        char *item;
+        assert(thegame.name);
+        item = snewn(9+strlen(thegame.name), char); /*ick*/
+        sprintf(item, "Help on %s", thegame.name);
+        menuitem = gtk_menu_item_new_with_label(item);
+        sfree(item);
+        gtk_container_add(GTK_CONTAINER(menu), menuitem);
+        g_signal_connect(G_OBJECT(menuitem), "activate",
+                         G_CALLBACK(menu_help_specific_event), fe);
+        gtk_widget_show(menuitem);
+    }
+
     menuitem = gtk_menu_item_new_with_label("About");
     gtk_container_add(GTK_CONTAINER(menu), menuitem);
     g_signal_connect(G_OBJECT(menuitem), "activate",
--- a/Recipe
+++ b/Recipe
@@ -95,6 +95,7 @@ Puzzles.dmg: Puzzles

 !begin am
 bin_PROGRAMS = $(GAMES)
+GTK_CFLAGS += -DHTMLDIR="\"$(htmldir)\""
 !end
 !begin am_begin
 GAMES =