Unless you're using a GPS specifically made for surveying, "several meters" is typical error for GPS, particularly in or around buildings. I trust Google Earth more than I trust the lousy receiver in a phone.
Well, I'm not sure about the GPS in a phone, I might try it at some point but I think it would likely prove to be too much work for too little accuracy gain. I was using google maps to get the value of 300m for the distance between two points, I just tried again on Google Earth Pro (hopefully more accurate), and I got 300.5m going from (approximately) the middle of the intersection, and by measuring the "long" and "short" distances (farthest two corners, closest two corners), I get an error of about 20m for the scale I'm basing my calculations on. This yields a total tunnel length of about 4 - 4.6km. I will likely try this in the future with more intersections to get a more accurate result in the future.
Again, I'll look into the phone GPS thing but I'm pretty sure Google Earth will be pretty accurate.