Hello anonymous user, Welcome to the Microsoft Q&A platform.
As per this common design principle guide In order to allow the ExpressRoute virtual gateway to advertise routes from Hub to Spoke, you must select the “Allow Gateway Transit” option in the VNet peering panel of the Hub. And then to allow the Spoke VNet routes to transit out of the ExpressRoute gateway, you must select the “Use Remote Gateway” option in the VNet peering panel of the Spoke. Once complete, all on-prem BGP routes advertised through ExpressRoute will be visible in the Hub and all participating Spokes, and vise-versa. You can also go through this FAQ documentation for additional details.
Hope this helps. Please let me know if you have any additional questions. Thank you!