docs: add a Gandr custom-voice integration guide

fern/customization/custom-voices/gandr.mdx

@@ -0,0 +1,49 @@
+---
+title: Gandr
+subtitle: Use Gandr voices in Vapi through the custom-voice provider
+description: Wire Gandr, a text to speech API for voice agents, into Vapi with the custom-voice provider and Custom Credentials.
+slug: customization/custom-voices/gandr
+---
+
+Gandr is a text to speech API built for voice agents. It reads numbers, dates, order IDs and addresses correctly, one engine speaks 23 languages with six voices, and every render carries an inaudible watermark. Audio streams back as it is generated.
+
+Gandr's `/v1/vapi` endpoint implements the custom-voice contract described in the [Custom TTS guide](/customization/custom-voices/custom-tts): it reads the nested `message` envelope, honours the requested `sampleRate` (8000, 16000, 22050, 24000 or 44100), and returns raw 16-bit little-endian mono PCM.
+
+<Note>A Gandr account and API key are required. The free key starts at 50,000 tokens.</Note>
+
+<Steps>
+  <Step title="Create a Gandr key">
+    Sign up at [gandr.ai](https://gandr.ai) and copy your API key.
+  </Step>
+  <Step title="Store the key in a Custom Credential">
+    In the [Vapi Dashboard](https://dashboard.vapi.ai), create a **Custom Credential** holding your Gandr key, following [server authentication](/server-url/server-authentication), so the key never travels in the assistant config.
+  </Step>
+  <Step title="Add Gandr as the voice provider">
+    In the assistant config, set the voice provider to `custom-voice` and point it at Gandr:
+
+    ```json
+    {
+      "voice": {
+        "provider": "custom-voice",
+        "server": {
+          "url": "https://tts.gandr.ai/v1/vapi",
+          "credentialId": "<your-custom-credential-id>",
+          "timeoutSeconds": 10
+        }
+      }
+    }
+    ```
+  </Step>
+  <Step title="Pick a voice and language">
+    Gandr's stock voices are `gandr-ava`, `gandr-dane`, `gandr-jenny`, `gandr-leo`, `gandr-lewis` and `gandr-mia`. The default is an English female voice. Any of the 23 languages can be set per request, independent of the voice.
+  </Step>
+  <Step title="Test the call">
+    Run a test call in the dashboard. If you hear audio, you are done. If the call times out, check that the Gandr key in the Custom Credential is active and that `https://tts.gandr.ai/v1/vapi` answers.
+  </Step>
+</Steps>
+
+## Pricing
+
+Gandr is $10 per million tokens, or $150 a month for an unmetered stream (one connection held open around the clock). The free key carries 50,000 tokens.
+
+Full API reference: [gandr.ai/docs](https://gandr.ai/docs).

fern/docs.yml

@@ -277,6 +277,8 @@ navigation:
                 path: customization/custom-voices/playht.mdx
               - page: Cartesia
                 path: customization/custom-voices/cartesia.mdx
+              - page: Gandr
+                path: customization/custom-voices/gandr.mdx
           - page: Custom transcriber
             path: customization/custom-transcriber.mdx
             icon: fa-light fa-microphone