As Dr.Molle pointed out <LineString><coordinates>1,1 3,3</coordinates></LineString>
works perfectly when you place it in the location field of your GFT. You can even place multiple LineString segmets and it'll treat that as a single entity:
<LineString><coordinates>1,1 3,3</coordinates></LineString>
<LineString><coordinates>2,2 4,4</coordinates></LineString>
Broken into two lines for readability here, but GFT treats all whitespace (newlines, tabs, spaces) as a single space, so copy-pasting the above into GFT will work too (except if you try to insert it into an SQL Query, then you'll need to replace the newlines yourself).