When I was at the University of Maryland at College Park in the late 1970’s I met Joe Weber and Charles Misner and others who were starting gravitational wave detection. So I have followed it over the years. I think it very unlikely that aliens would be using the very narrow band of electromagnetic signals you have chosen.
Is there anyone who is looking? If you dig, every frequency available to electromagnetism is also being investigated for gravitational signals. But I have not seen anyone looking for evidence of coded signals in the gravitational bands. I have tried to keep track of all the gravitational sensor groups, especially low cost, high sampling rate, time of flight methods that all countries, and small groups can afford. Plus expecting arrays will be more effective. SETI might be at this for centuries, so building a broad base in all countries makes sense.
If someone has already made efforts to organize gravitational searches, I would like to see what they did.
Related to that, I was wondering if you are sharing all frequencies in the small electromagnetic array you built. The band from nanoHertz to GigaHertz is the most likely. Much of that is treated as noise, because no one has spent time to look closely. LIGO has changed their sharing policy a bit, partly because I was bugging them to share their earth and solar system data. Joe Weber and Robert Forward had a different view of gravitation. That it should be used for imaging, communication and power. I expect a society based on gravitational systems will not use noisy electromagnetic signals where most of the power is wasted in broadcasts or channel losses. You cannot do good gravitational work if all the sources of noise are not traced, characterized, mapped and then used as reference.
Richard Collins, The Internet Foundation