Gnus usually moves point to a pre-defined place on each line in most buffers. By default, point move to the first colon character on the line. You can customize this behavior in three different ways.
You can move the colon character to somewhere else on the line.
You can redefine the function that moves the point to the colon. The
function is called gnus-goto-colon.
But perhaps the most convenient way to deal with this, if you don’t want to have a colon in your line, is to use the ‘%*’ specifier. If you put a ‘%*’ somewhere in your format line definition, Gnus will place point there.