The program declares a "gap" anytime that two endpoints are more than 0.00001" apart. This refers to two curves that do not meet as well as two curves that overlap. My approach to this problem is to zoom into where the gap is to see if it is a gap or an overlap. The next thing I do is use the selection arrow. With neither curve selected, draw a small box around the end of one curve and drag the small red box to the end point of the other curve. If you are zoomed in tight this may not work, so the next trick is to nudge the point over using the keyboard arrow keys, of course, the nudge distance will first need to be set to a very small value. The last trick is to use the "join" tool and join the two curves together. If you don't need the curves to be separate lines then you may try this first. Note that using the join tool with arcs will change the arcs to a spline, the last tool in the arcs palette will change the spline back to arcs if needed.