About Loading osm Files in GMap.NET

This article describes how to copy and use cache data.
They are all old articles, so they may have changed, but please try them.

GMap.NET+C#+postgreSQL without Internet connection
GMap.net offline

If you run the following demo programs, the cache data will be created locally.

https://github.com/radioman/greatmaps/tree/master/Demo.WindowsPresentation
https://github.com/williamwdu/GMap.NETChacher

By default, the cache data is located in the following location.
The en directly above may have changed depending on the settings.
USERNAME is the folder name for the logged in user.

C:\Users\USERNAME\AppData\Local\GMap.NET\TileDBv5\en\Data.gmdb

You can explicitly specify other cache locations.

usegmap.CacheLocation=@"C:\Users\xxx\Desktop\";to specify the cache location.

Simply copy the SQLite file to the same location as the default or explicitly specified.

Also, this article describes how to use cache data (only) in your own program.
GMap.NET explicit load cache?

Other
The Import/Export correspondence table (including tools) between the OSM and other formats is as follows:
Converting map data between formats

Here's a Q&A and article asking if GMap.NET supports Google's KML format.
However, the main developer is not supported.
How to import/export kml and display in gmap.net
gmap.net|C#|DXF|KML||robot|mission

Tutorial site introduced above (not sure if it will be helpful)
GMap.NET|Independent Software

Note: I'm not sure if it's possible, but as an idea.

You might want to build a copy of the OpenStreetMap server on your PC and then browse to GMap.NET.

It's old, but you may be able to use this article.
OpenStreet map server installation on windows
Local OSM server on windows
How to serve OSM mapnik files on Windows with Apache for leafLet.js
Are there any precanned OSM server sets for Windows that contain the database, data loader and anything else
How to build an OpenStreetMap on Windows/Mac (local environment)

For your information, there is an article that I set up on Ubuntu.
Create your own OpenStreetMap server (Install OpenStreetMap on AWS EC2/Ubuntu14.04)
Tried building an OpenStreetMap tile server