Question: In gravitational wave why is it important to understand mass?
About the article "Gravitational wave", section "Radiation from other sources".

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  The receiving end of the light transmission must be located at a higher gravitational potential in order for gravitational redshift to be observed.
In other words, the observer must be standing "uphill" from the source.
If the observer is at a "lower" gravitational potential than the source, a gravitational blueshift can be observed instead.
Tests done by many universities continue to support the existence of gravitational redshift.
Gravitational redshift is not only predicted by general relativity.
Other theories of gravitation require gravitational redshift, although their detailed explanations for why it appears vary.
(Any theory that includes conservation of energy and mass–energy equivalence must include gravitational redshift.)
Gravitational redshift does not assume the Schwarzschild metric solution to Einstein's field equation – in which the variable formula_23 cannot represent the mass of any rotating or charged body.  
A gravitational-wave detector is any device designed to measure gravitational waves, tiny distortions of spacetime that are predicted by Einstein's theory of general relativity.
The existence of gravitational radiation is a prediction of general relativity.
Gravitational waves are perturbations in the curvature of spacetime caused by accelerated masses.
Since the 1960s gravitational-wave detectors have been built and constantly improved.
The present-day generation of resonant mass antennas and laser interferometers has reached the necessary sensitivity to detect gravitational waves from sources in the Milky Way.
As of early 2014, no direct detection of gravitational waves has yet been accomplished, although a number of experiments have now added to the evidence that gravitational waves are more than mathematical anomalies of relativity calculations (ref.
the 1993 Nobel Prize in Physics).
On 17 March 2014, astronomers at the Harvard-Smithsonian Center for Astrophysics announced the apparent detection of gravitational waves, which, if confirmed, may provide strong evidence for inflation and the Big Bang. 
The local conservation of energy, and the consequential variation in rest mass, demand that gravitational mass is treated under the de Broglie wave theory.
Mass is defined by the DeBroglie frequency of that particle.
The red shift caused by the curvature of space-time, a time dilation expressed by the metric component formula_28, is suffered not only by the photon but also by the atom with which it interacts and is thus undetectable.
The red shift that is detectable is caused by the increase in rest mass that fundamental particles undergo when raised to the higher level.
Gravitational red shift in this theory is interpreted not as a loss of gravitational potential energy by the photon but as a gain of gravitational potential energy by the apparatus measuring it.
The red shift predicted is hence equal to the difference in Newtonian potential and thus identical with that of GR as confirmed in the Pound–Rebka experiment. 
The DECI-Hertz Interferometer Gravitational wave Observatory (or DECIGO) is the proposed Japanese, space-based, gravitational wave observatory.
The laser interferometric gravitational wave detector is so named because it is to be most sensitive in the frequency band between 0.1 Hz and 10 Hz (a decihertz).
It is due for launch in 2027. 
An atomic gravitational wave interferometric sensor (AGIS) is a novel detection scheme to detect gravitational waves, proposed by S. Dimopoulos et al.
in 2008.