使用单个存储库，您可能会遇到从CI / CD管道部署的困难 - 例如，当仅更改另一个服务时，不必要地部署到一个服务。
对于多个项目与多个服务，这可能有所帮助：在GAE项目/应用程序级别与服务/模块级别实施CI / CD环境的优势？
您提到的静态网站链接不是GAE的一部分，它是GCS的一部分- 一种不同的GCP产品。它可以单独使用 - 对于静态网站，但可能很难/不可能：
I'm developing my first node.js app deploying to GAE.
It'll be organized as an API service and a front-end web app developed with Next.js
I'm looking at this architecture, and, although I have the app separated in two repositories I could have one merged repo to create two different microservices:
For me, it seems overwork creating a new repo to merge them and deploy (doesn't it break one of the basic ideas of microservices to make isolated deploys?)
I have to discourage this because we need SEO in some of the parts, and We should use Next.js (or similar):
Another idea I've been working on is... create different GAE projects for front and API to deploy independently. For me, it seems like the best option, but I would like to know your opinion as GAE experts.
Which one should I use?
GAE doesn't care how is the code to be deployed into the services mapped to one or more VCS repositories (or no repositories at all). That's entirely up to you.
With a single repository you may encounter difficulties deploying from CI/CD pipelines - for example unnecessary deployments to one service when only the other one is changed.
Many examples out there focus on applications rather than services, but those are nothing more than the default services of those applications. Personally I like keeping the code for different services in separate directories, see the image captured in Can a default service/module in a Google App Engine app be a sibling of a non-default one in terms of folder structure? (it's no longer present on the updated documentation page). This also allows for easy mapping to multiple, separate VCS repositories
As for multiple projects vs multiple services, this might be of help: Advantages of implementing CI/CD environments at GAE project/app level vs service/module level?
The static website link you mentioned isn't part of GAE, it's part of GCS - a different GCP product. It's fine to use by itself - for a static website, but it might be difficult/impossible to: