You can switch to any name during N-400, theoretically even to Arnold Schwarzenegger if you really wanted to.
You can certainly switch to Rohit Gandhi if you prefer. After naturalization in the US you'll lose your Indian citizenship.
However, moving forward, in rare cases you'd have to explain name discrepancy between current name and name in birth certificate. You'll have copy of naturalization certificate to explain it.
Your current name is Rahul Gandhi.
FNU stands for "first name unknown". So your GC should have been corrected as soon as you got it.
I wouldn't think it's a biggest deal, can be explained / corrected during N-400 if you need.