PowerApps - Canvas apps - Merge 2 lists in a gallery

Let's Say you have 2 SharePoint Lists

ParentList

-ID -Name -Details

ChildList

-ID -ParentID -Comment

Assume you want to display a list of comments with the name of the associated Name from the Parent List in a gallery

Set the Gallery Items to

AddColumns(
    'ChildList',
    "ParentInfo",
    Lookup(
        ParentList,
        ID='ChildList'[@ParentID]
     )
)

This way, in the Gallery, for each row you will find a new field of type record named "ParentInfo". so you can add columns as 'ParentInfo'.Name