When using personalisation fields in Taxi, the GCP connector is platform agnostic, meaning after you export to it from Taxi, you can do what you need with the file. E.g. transfer over to your own sending tool. Therefore you can use any language for personalisation variables you need.
These can be set up in the personalisation settings in Taxi.
These will become available in rich text fields in the editor.
You can add as much or as little code here as you like, so it’s possible to use your templating language to do things like use a default name:
{% if ${first_name} == blank %}Hi there{% else %}Hey {{${first_name}}}{% endif %}