When MacGist is first launched, this is what you'll see when you click on the menu bar icon.

To use MacGist, enter in your GitHub credentials into the Preferences pane (credentials are stored locally, never transmitted to anyone besides GitHub).

For now, you'll have to quit and relaunch MacGist to get the menu to refresh (working on that, this is a beta!)

Select the gist you want to target paste actions to.

Enter ⌃ ⌥ ⌘ V (Control-Option-Command-V)

And you'll see it appear in your default gist!