dynamodb-toolbox | simple set of tools for working with Amazon DynamoDB | Object-Relational Mapping library

I do not know how can I create GSI

This is on the documentation, more specifically available here.

I got one big problem that if I want to get all posts which got a special category. Now I'm using a scan and contains a filter.

In this case and based on your data model currently, you need to configure a GSI with category being the partition key and id being the sort key, where you need to configure the primary key as being the composite primary key of PK and SK, as described here.

This will allow you to query the data by category on that GSI, as you have the category as the partition key, you will hit one partition when you perform your query.

how can I use it in query width DocumentClient and node.js

I suggest that you figure these out first, and make it work in isolation w/o worrying about your programming language constraints. Once you get a handle of this, you can worry about programming language challenges in isolation.