As well as being able to enhance screens/tabs and adding new screens to the GMGRANT transaction you can also enhance the events such as pressing save, enter, cancel, back etc. This is implemented by enhancing there associated event codes such as ISSTA, ISDATA, XCHG, DCHCK, DSAVC etc within transaction GRANT_CUSTOMIZE(BDT). A list of the different events and there uses can be found here
This functionality can be very useful, one use for this would be if you wanted to create a whole new tab/screen which saved data to a ZTABLE, but as with the standard tabs you needed to ensure that the data was not stored until the user clicked on the save button. You would therefore need to add the actual save functionality to the save event (i.e. DSAVC). To do this you simply add a function module which has been created in your function group to the DSAVC event. Another useful event is DCHCK which gets called when the user presses the enter button, this allows you to add any extra field validation which needs to be done before the transaction gets to the save functionality. See below for steps on how to implement the event enhancement:
Execute grants management BDT development transaction (GRANT_CUSTOMIZE)
Select Grant Master BDT Customizing->Events(GMS7)
Select the event you want to enhance(i.e. DSAVC)
Double click on the 'Event-> Function Modules' folder option from the left column
Select 'New Entries' button. Enter an item number and the name of your custom function module, other fields should be same at Standard SAP entry.