In Revit API, Grids of all types are represented by the DB.Grid. The Grid primitive in Rhino.Inside.Revit represents a Grid.
Pick Existing Grid
The Query Grids component provides a list of all the Grids in the project. These can be refined using the Name, Elevation or Filter inputs.
You can also use the Grid Parameter to choose an existing grid. Right-click the Grid primitive gives access to various Grid functions.
Querying Grid Types
See Modifying Types for getting Grid Type Information.
Extract Grid Curve
Passing a Grid, to a Grasshopper Curve component, will extract the curve of that grid:
The Component Add Grid+ v1.0 requires a Line or Arc, Type & Name inputs are optional.