The ArcGIS Grid type allows you to display a grid over a 2D map using one of the predetermined grid types, such as UTM, USNG, Latitude/Longitude, and MGRS. The QML sample shows you the various visual properties you can modify in the grid - https://github.com/Esri/arcgis-runtime-samples-qt/tree/main/ArcGISRuntimeSDKQt_QMLSamples/DisplayInf...
Based on what you are explaining, I'm unsure if this grid type is what you are needing, or if you are needing to create your own graphics using the boundary coordinates. Could you explain your workflow in more detail please?