A GDO is a useful little test instrument for finding the resonance of a tuned circuit. These tuned circuits might be in a VFO or perhaps a trap for an antenna. Many years ago I had a commercial version that was quite large and had a large tuning dial. (The plug-in coils deteriorated over the years). Recently I ordered a modern version in kit form from HecKits.
The instructions are good although I found that the board deviates slightly from the image in the instructions and, as there are no component overlays, it can be a little tricky to figure out where each component goes.
The board went together quite smoothly, with just a little head scratching. Clearly there are a few changes since the manual was written but all worked on first power up for me. The kit comes with the lowest frequency coil pre-wound, which makes it easy to test.
In the end the kit all worked and fits into a nice compact case. I made one error in that I reversed pin 1 and 3 in a coil I wound and consequently had trouble getting it to oscillate. It didn't take long to figure out the problem and reverse them.
As well as finding resonance in tuned circuits, this device is a handy small signal generator. The kit isn't cheap - US$100 plus some extra postage to me. My kit came with some spare multi-turn pots for some reason.