Related github issue https://github.com/karthink/gptel/issues/346

gptel.el.diff

 (defun gptel--strip-mode-suffix (mode-sym)
   "Remove the -mode suffix from MODE-NAME.
 
 MODE-NAME is typically a major-mode symbol."
-  (let ((mode-name (thread-last
-                (symbol-name mode-sym)
-                (string-remove-suffix "-mode")
-                (string-remove-suffix "-ts"))))
-    (if (provided-mode-derived-p
-         mode-sym 'prog-mode 'text-mode 'tex-mode)
-     mode-name "")))
+  (cond
+   ((eq mode-sym 'enh-ruby-mode) "ruby")
+   ((eq mode-sym 'js2-mode) "js")
+   ((eq mode-sym 'scad-mode) "openscad")
+   (t
+    (let ((mode-name (thread-last
+                       (symbol-name mode-sym)
+                       (string-remove-suffix "-mode")
+                       (string-remove-suffix "-ts"))))
+      (if (provided-mode-derived-p
+           mode-sym 'prog-mode 'text-mode 'tex-mode)
+          mode-name "")))))
+

@dgutov thank you!

And your question about a better way: unfortunately we don't have an unequivocal major-mode=>language mapping anywhere yet, but there is another source of information: major-mode-remap-alist and (most recently) major-mode-remap-defaults. So one could also check whether the current mode is associated as a value with some major mode as a key which fits the traditional naming scheme (when the mapped one doesn't).

But I would only bother with it after finding a real-world mode that doesn't fit the current detector. That mapping is only used for TeX/LaTeX and ts modes by default.