docs: add service_tier to gemini docs

docs/my-website/docs/providers/gemini.md

@@ -65,14 +65,13 @@ response = completion(
 - modalities
 - reasoning_content
 - audio (for TTS models only)
+- service_tier
 
 **Anthropic Params**
 - thinking (used to set max budget tokens across anthropic/gemini models)
 
 [**See Updated List**](https://github.com/BerriAI/litellm/blob/main/litellm/llms/gemini/chat/transformation.py#L70)
 
-
-
 ## Usage - Thinking / `reasoning_content`
 
 LiteLLM translates OpenAI's `reasoning_effort` to Gemini's `thinking` parameter. [Code](https://github.com/BerriAI/litellm/blob/620664921902d7a9bfb29897a7b27c1a7ef4ddfb/litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py#L362)
@@ -298,6 +297,16 @@ curl http://0.0.0.0:4000/v1/chat/completions \
 
 
 
+## Usage - `service_tier`
+
+LiteLLM propagates OpenAI's `service_tier` parameter to Gemini, and also extracts it from the response headers (`x-gemini-service-tier`) into `model_response.service_tier`.
+
+| OpenAI `service_tier` | Gemini `service_tier` | Notes |
+| --------------------- | --------------------- | ----- |
+| `"auto"`              | `"priority"`          | LiteLLM maps OpenAI's `"auto"` to Gemini's `"priority"` tier, as `priority` will fall back on Gemini. |
+| `"flex"`              | `"flex"`              | Direct mapping. |
+| `"priority"`          | `"priority"`          | Direct mapping. |
+| Any other value       | Passed as-is          | Unrecognized values are sent blindly so that the provider can reject the request directly if invalid. |
 
 
 ## Text-to-Speech (TTS) Audio Output