This latest version 04.03.01.05 identifies itself as "00.04.03.SP1" in Utility>System>System Info.
However, you will get more information about the software/firmware your scope is running, if you print a "Param" .txt file from the Storage menu. What we are calling "firmware" here is what Rigol calls "SoftWare" in the Param listing... and what Rigol calls "Firmware" refers to some other bit of the scope's operating system.
For example, here is the first section of a Param listing from my scope (serial number redacted):
Model:DS1xxxZ
SN:DS1ZA170xxxxxx
Manufacturer:RIGOL TECHNOLOGIES
Board Ver:0.1.1
Firmware Ver:0.2.3.11
BOOT Ver:0.0.1.2
CPLD Ver:1.1
SoftWare Ver:00.04.03.SP1
As far as I am aware, only the "SoftWare Ver" changes when a new .gel file (what we have been calling "firmware") is installed.