Disclaimer: I'm one of the co-founders of Shuttle. Also, I haven't used Pulumi so I don't want to misrepresent it.
However, as far as I understand, Pulumi is an infrastructure _as_ code solution, offering an SDK in various languages which wrap providers enabling you to define your desired infrastructure. In the context of a cloud provider like GCP, this means wrapping the existing GCP primitives and services (i.e. GKE) and enabling you to declare your desired infrastructure in your favourite programming language.
Shuttle is an infrastructure _from_ code solution. The infrastructure that is provisioned for you is defined implicitly by your application's code. Static analysis is done at compile time to figure out what you need implicitly (i.e. if you're using a database connection pool, you probably need a database). Furthermore Shuttle offers its own primitives (i.e. secrets management) without a necessary correspondence to an underlying cloud provider (although there are some, like AWS RDS).
However, as far as I understand, Pulumi is an infrastructure _as_ code solution, offering an SDK in various languages which wrap providers enabling you to define your desired infrastructure. In the context of a cloud provider like GCP, this means wrapping the existing GCP primitives and services (i.e. GKE) and enabling you to declare your desired infrastructure in your favourite programming language.
Shuttle is an infrastructure _from_ code solution. The infrastructure that is provisioned for you is defined implicitly by your application's code. Static analysis is done at compile time to figure out what you need implicitly (i.e. if you're using a database connection pool, you probably need a database). Furthermore Shuttle offers its own primitives (i.e. secrets management) without a necessary correspondence to an underlying cloud provider (although there are some, like AWS RDS).