Using which tool? In Gimp 2.6.8 in Ubuntu 10.04, I don't see any tool which I could use for drawing lines unless I open GFig.
the answer is snip from <<Yohan Pereira>> reply
" actually its not so obvious but select any drawing tool (like paint brush or pencil). click (and release) at the place you want to start the point, then hold shift and click on the desired end position .... tada!!! "