doc(frontend-python): Additional information about GPU wheels install

docs/get-started/installing.md

@@ -23,6 +23,12 @@ pip install -U pip wheel setuptools
 pip install concrete-python[full]
 ```
 
+{% hint style="info" %}
+Not all versions are available on PyPI. If you need a version that is not on PyPI (including nightly releases), you can install it from our package index by adding --index-url https://pypi.zama.ai/cpu.
+
+In particular, wheels with **GPU support** are not on PyPI. You can install it from our package index by adding --index-url https://pypi.zama.ai/gpu, more information on GPU wheels here.
+{% endhint %}
+
 {% hint style="info" %}
 The full version requires [pygraphviz](https://pygraphviz.github.io/), which depends on [graphviz](https://graphviz.org/). Make sure to [install](https://pygraphviz.github.io/documentation/stable/install.html) all the dependencies on your operating system before installing `concrete-python[full]`. 
 {% endhint %}