OK, I hacked a cheesy Chinese USB to micro USB cable, and added 6 feet of nice 18AWG wire into it. I used only about 3" of the Chinese cable on each end to make the connections. It has extremely fine wire. I am not sure of the gauge, but I use a pair of Klein 11057 precision wire strippers to work with this fine wire, and it has notches for all the way down to 32AWG. I stripped the first tiny wire with those 32 AWG notches just as a test, and I do not think I cut the copper core. I moved up to the 30 AWG to do the other 3 wires.
After soldering and heat shrink assembly for support of the tiny wire connections I am seeing between 4.88 to 4.92V on the Gertboard 5V pins. (It obscures one of the Pi power TPs, so it is easier to clip micro clip leads to the pins on the Gertboard itself for monitoring).
Based on all your advice, this will likely work OK.
One added note I discovered in this project - The Amazon Basics 6 ft. gold connector cable I just bought has stamped right on it "28 AWG/1P AND 28AWG/2C". This indicates that ALL the wires within the cable, both for power and data, are 28AWG wires.
Some USB cables, including the 3 foot cable that ships with the CanaKit version of the Pi, have the following marking : "28AWG/1P + 24AWG/2C". These cables use 24AWG wires to carry the power in the cable. One reason it has less line loss.
What would really be nice would be to find a USB cable that would have even heavier gauge wire. But I don't know if anyone makes such a product.