Answer: Both of these are very large trees, so I do not think 50 feet is far enough apart for each tree to have the space it really needs to grow and develop to its full potential.
The ginko can easily be 50 feet tall (sometimes 100) and 25 feet or more wide, while the walnut would likely be in the 75 foot range (sometimes larger) with a spreading crown about as wide as it is tall. This means these are large trees better suited to a very large scale lawn area or perhaps a public area such as a golf course or park.
There is also the possibility of the juglone in the walnut roots inhibiting growth of the gingko. Unfortunately, in a brief search I am unable to locate any reference to gingko sensitivity to black walnut trees.
Black walnut trees can send their roots out as far as three times their height, and the roots will be competition for the gingko tree so the wider apart you plant them the better. I would suggest at least 100 feet, more if possible.
Q&A Library Searching Tips