Thank you for reaching out.
I followed the example here and was able to set the property
Propagate gateway routes to yes.
From the snippet above not sure about where you have defined the routetable resource.
resource routetable 'Microsoft.Network/routeTables@2021-02-01'
Can you follow the example above and see if that resolves the issue?
if not, can you add the whole bicep script? for additional troubleshooting. Thank you!