Retrieve data from Google Analytics by calculating a dimension

Hi Giuseppe89 

If you don't want to import your data. 
You can use the REST API in Power Query, which is a viable way to do it at the moment:

Google Analytics Data API  |  Google for Developers

You can use the API to get data from data source and write mcode to get the output data you want.

Here is a sample for your reference, and this code needs to be modified:

let
    Source = Web.Page(Web.Contents("YOUR_API_ENDPOINT", [Headers=[Authorization="Bearer YOUR_ACCESS_TOKEN"]])),
    Data = Source{0}[content],
    JsonData = Json.Document(Data),
    #"Converted to Table" = Record.ToTable(JsonData),
    #"Added Custom" = Table.AddColumn(#"Converted to Table", "Category", each if Text.Contains([PagePath], "category_a") then "A" else if Text.Contains([PagePath], "category_b") then "B" else "Other"),
    #"Grouped Rows" = Table.Group(#"Added Custom", {"YearMonth", "Category"}, {{"TotalUsers", each List.Sum([Users]), type number}})
in
    #"Grouped Rows"

Here for your reference:

javascript - Google Analytics Reporting API V4, User does not have sufficient permissions for this profile - Stack Overflow

The above is an implementation that requires you to have a deep understanding of Power Query M code and understand the relevant knowledge of API.

Best Regards

Zhengdong Xu
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