Edit: If your question was mainly about inserting links in gg, just highlight the text you want to link, then click on the word "Link" in the toolbar. Then type or paste in the address you want.
Quick test.
This was copied from the rendered level of a Tiddler and pasted to GG.
Kinda work ... But loose line spacing.
UPDATE: Actually on posting the line breaks came back ...
This is a bold test.
This is an italic test.
This is underlined
This is a subscript test
This is a superscript test
This is struckthrough test
This a Google Groups link test
Don't work ...
This is a TW indented quote
Here is some inline code
Here is a block
of codeThere is an extension called "markdown-here" which will let you toggle markdown text inside gmail and GG. (Haven't tried it myself).