i just put gibson daul exstreme cat back system on my truck now it sounds like flutters or even has a chopper like sound when yur at really low rpm,s and not when yur hittin gas

 

Sounds like you've got the dreaded drone.
What year model and engine do you have?
Did you replace the entire factory exhaust from the y pipe back?

 

2008 1500 quad cab with a 5.7 hemi its a cat back system.

 

Yeah , that's your MDS kicking in that is causing that pulsing "helicopter flying under water" drone.
Very common but still annoying. The 08 oem exhaust had 1 resonator on the tail pipe to suppress the drone your hearing.
I assume you had a shop install the new exhaust and it's a single in duel out system.
The only way to eliminate the drone is to disable MDS with a tuner or drive around town w/your O/D off. or install a couple of resonators after the muffler.