Gravtiational waves don't have any specific frequency, and it's definitely not easy to calculate the frequency though any simple equations. The frequency of the gravitational wave depend on the sources they originate from. For instance, gravitational waves originating from a pair of dense stars would have a frequency ranging from 10 to the power of -3 Hz all the way to 100 Hz, or a pair of supermassive black holes would have a frequency ranging from 10 to the -10 Hz all the way to 10 to the -2 Hz. As you can see that is quite a large range, so I'm sorry but it would be extremely difficult to calculate this value. On the other hand, if you want to know more about gravitational waves, there are many books containing great information, so I would say visit your local library and hope that they have books on them. If not, you may want to find an eBook or an online source.