Do the outer gas planets radiate their mass? We know that the sun experiences angular momentum loss, and radiates a portion of it's mass (though helicity is conserved). Can we say the same about massive Jupiter, or even Saturn, Uranus or Neptune? I'm thinking specifically about the contribution this might have to the Interplanetary Medium (IPM)...
 A: All bodies with temperatures higher than 0Kelvin radiate away electromagnetic energy according to black body radiation. The power emitted goes according to the Stefan-Boltzmann law.


The  law states that the total energy radiated per unit surface area of a black body across all wavelengths per unit time (also known as the black-body radiant exitance or emissive power),, is directly proportional to the fourth power of the black body's thermodynamic temperature T.

According to both special relativity and general relativity mass and energy are related, so bodies radiating electromagnetically away their energy are loosing mass. Thus the general answer is yes.
Now the energy lost in this way by planets whose temperatures are very much smaller than the stars is off course very much  smaller , due to the fourth power of the temperature.
Stars also lose mass due to large ejections of plasma which does not happen with planets. 

the contribution this might have to the Interplanetary Medium (IPM)

Radiation leaves the interplanetary space very fast.
If you are talking of particulate mass ejections, the planets will be losing molecules  from their outer atmosphere at a much smaller rate than the plasma ejections from the sun. After all planets do lose their atmosphere. and that matter will be in the interplanetary medium , joining the particulate ejecta from the sun.
A: Yes. There are a few ways this happens. Ultimately, it's an energy budget issue. There is less "mass" being radiated, and more "energy" being radiated. Of course, they're fundamentally related. 
Here are some of the ways this can happen: 


*

*Loss of energy due to tidal dissipation 

*Radiative Heat Transfer / Radiant heat loss 

*Atmospheric particle escape (Jeans Escape) -- actual mass loss

*Hydrodynamic Escape  -- actual mass loss

