GitHub Flavored Markdown does not support merged tables. However, GitHub does support a special subset of HTML (GitHub HTML?) that allows you to build complex, tables with merged cells.
Here is a demo on GitHub.
The table was exported using Box Table Baker for Google Sheets, which allows you to export selections of a spreadsheet as HTML tables.
Google Sheets does not allow export of selected spreadsheet ranges. Its built-in HTML export feature only downloads all the sheets within the spreadsheet as a “web page” to a zip file.
Box Table Baker adds several features that improve productivity. The most important feature for this post is the export of formatted merged tables, which are supported by GitHub HTML.
Bold, italic, and strikethrough font styles for each spreadsheet cell are exported to “GitHub HTML” tags (e.g., <b>, <i>, and <del>, respectively). Also, monospace fonts are detected and exported within <code> tags.