I don't have the fuel gauge but I haven't heard any comments on it being hard to install, I'll bet its a simple bolt to the bars... lift the tank... plug in harness kind of deal. The only problem that seems to come up is that the original fuel sending unit float arm in the gas tank is not adjusted correctly and makes the fuel gauge inaccurate. If that becomes the case you have to remove the sending unit from the bottom of the tank and adjust it. If your bike is still under warranty have the dealer do it. I'll think positive and we'll just say yours is adjusted perfectly from the factory.
Welcome. The fuel gauge is OK installation-wise. But... it's as reliable as the idiot light. Meaning... that the in-tank fuel level sensor (a float arm, basically) that electrically feeds both instruments has to be set-up properly ex-factory. This was NOT the case on many or most 2004-05 Rockets, hence this note to you. As Tomo suggests, the fix is known to dealers (but not always volunteered by them ) and easy to perform. But if you get wrong signals, i.e. idiot light "on" and gauge suggesting an empty tank way too early, it would simply mean that the fix has not been done on your 2005 issue. And that it needs to be done now.
(certainly hope this does not read as oldtimer's gaga )