What markup does the gwidgets glabel use in R?

In the glabel gWidget object, it is possible to use markup in the label text. I have activated this to true and tried to use multiple markup conventions eg. for fat, '' 'text' '' Conventions, etc. But none of them work. Googling "What's the markup code in gwidgets" didn't help me either. I would like to format some of my glasung text in bold and centralize other parts.