Does the GRACE output in "cm" shows the anomaly change in TWS?

Yes

How we can find the GWS change in unit using GRACE with the help of GLDAS data?

You can use this equation

GWS=TWS-SM-SWE-CWS

SM is soil moisture

SWE is snow water equivalent

CWS is canopy water storage.

SM, SWE and CWS can be obtained from GLDAS.

You can use tis formula for convert

1 kg/m2=1.00 mm H2O

One more question about the GWS. As we have TWS for every month and also the SWE, SM and CWS for monthly data. Now for those data, we need to change every month. I mean ΔTWS and also the other ΔSWE, ΔSM and ΔCWS for GWS. Because the original data have non-negative values for (SWE, SM and CWS) but in many papers, I can see the variation between positive and negative values also.

Muhammad Zeeshan Ali You need to make anomaly of each variable of GLDAS data similar to GRACE data to estimate GWS using GRACE TWS. The GRACE base line is 2004-2009.

GRASE product is the exact TWS deviation. and according to GLDAS guidebook, you can estimate GW by this formula:

GWS = TWS - RootZoneSoilMoisture - SnowWaterEquivalent - CanopyInterception.

however, a care must be taken as GLDAS models do not include surface water storage (such as rivers, dam/reservoir, and lakes).  Also, GRACE LWE is a deviation from the baseline mean (2004-2009)

The time-variable component of the gravitational field is provided by GRACE and can be defined or represented in terms of changes in the total amount of water stored. Surface water, groundwater, snow water, canopy water, and soil moisture all contribute to total water storage. For your area of interest, you must determine the necessary water storage containers. After that, delete the estimations for these water compartments from the GRACE total water storage change. For a semi-arid environment, for instance, GW = TWS - Soil moisture - Surface water - Canopy water.