Can gravitational waves produce photons?, How could these photons be discovered, and can this be a new proof for the existence of gravitational waves? In a recent paper (arXiv:1610.02973 [gr-qc]) written by two U.S University Professors (from California State University Fresno and Embry Riddle Aeronautical University), theoretically, is has been shown that massless particles, such as photons, could be produced by the gravitational waves.
In particular they wrote:
"If the production mechanism of electromagnetic radiation fr om the gravitational wave background proposed here occurs and is significant, then we would predict that the gravitational wave signal should also be accompanied by an electromagnetic signal. However, in our process this electromagnetic signal should have roughly the same frequency as that of the gravitational wave. Thus we would predict that the electromagnetic wave coming from the gravitational wave would have extremely long wavelengths , on the order of 100s of kilometers i.e. the associated electromagnetic wave would have very large wavelengths. These wavelengths are of such a length that they could easily have g one undetected up to now."
Two particular questions which we may ask are:
1- How such large wavelength electromagnetic waves could be discovered?
2- Can discovering these sorts of electromagnetic waves be another proof for the existence of gravitational waves?
 A: Two answers for your two questions, but first let me say that I have not read the paper to ascertain its veracity or accuracy, nor the parameters besides wavelength (such as power and directivity). Perhaps you can expand on that. 
1) detecting radio waves with wavelengths (WL) of hundreds of kilometers
It's done already for communications with submerged submarines, and detections of other atmospheric and solar perturbations. The simplest antenna is just a long cable dragged behind the sub. It's obviously not hundreds of Kms long, more like maybe 100's of meters long.  It'll detect, but it is not a high gain, or even zero gain antenna, rather it is a lossy antenna. So it all depends on how much power is received. 
Note that 100 km WL is 3 KHz. That frequency range and even lower is called VLF (very low freq), and also low enough ULF. See the wiki article about ULF waves, it also has a picture of what is needed to generate them to communicate with the subs. There's been other detectors or antennas for VLF and ULF, usually a distributed set of antennas over a large area. 
The whole question is what is the power level that it must detect, and how does it make sure that it is from gravitational waves and not from the sun, or magnetic effects on the earth etc, i.e., how is it unique. The  power level will be the critical parameter. 
Since EM waves interact more strongly with matter than gravitational waves, even if they are some orders of magnitude weaker (I power, at the antenna), it might still be easier to detect them. The uniqueness is another question.
2)another proof of gravitational waves? Don't need it, it's already been detected indirectly (which this would also be) and directly. But it would be additional evidence, and it might also tell us something about the gravitational wave parameters that would supplement what we observe directly. 
Specifically, if we can get the direction of arrival from a combination of 3-4 detectors, we could locate the source, but that's also going to be a similar issue than what we need for that from gravitational waves because they have a similar wavelength (for direction finding) and the same speed (for time difference of arrivals), so about the same accuracies. Still, that could be used to correlate them, in direction or location and time. 
