Hi Anil,
It's Already Modified. It's Field-Symbol Poperties.
Field-Symbol 's are using the Pointer methods, so after call the ASSIGN COMPONENT statement with respect to Field Name, system take the memory area of the field.
So whatever calculation you do on <lw_amnt>, it will store into the <fs_gwa_tab>-QTY.
Field Symbols - ABAP Programming (BC-ABA) - SAP Library
https://help.sap.com/saphelp_nw70/helpdata/en/fc/eb387a358411d1829f0000e829fbfe/content.htm
Reagrds.
Praveer.