What does it mean by "GCP is built for developers"
From what I understood, GCP makes it easier to do certain type of work done by developers. Also, in certain functionalities, it requires its users to write code as well, so development background is needed. This resource given with previous chapter video makes the differences between AWS and GCP clear. https://read.acloud.guru/serverless-superheroes-lynn-langit-on-big-data-nosql-and-google-versus-aws-f4427dc8679c
The way I see the statement "GCP is built for developers" is that all the services in GCP are built and designed in Developer friendly way. They kind of natively fit into the desired solution which you as a developer/architect want to build.
The general idea when working with GCP is to develop first and then think of tooling later.
Majority of the services requires minimal configurations (that too based upon on the requirements of the application) and all the internal infrastructure complexity is abstracted from you as a developer.
I developed a Spring Boot microservice, later which I was able to deploy it to Cloud Run, GKE, App engine and used a cloud build as CI/CD. Which was quite convenient and when I was developing did not need to think of the tool chaining initially.
Also as Harshvir specified above some of the GCP services do expect users to have a little developer background to use it effectively.