Unless you are an EXTREME hardgainer you should be doing some type of CV training.....period! And even if you are an extreme hardgainer and are over 35 you should still be doing cardio. Ken Leistner stated it perfectly when he said:
Nobody ever dies because their arms are not big enough, but lots of people die every day because they are not in good enough CV shape. If you don't gain well, 2 sessions a week is probably your best bet. Most people that gain at a reasonable clip can handle 3 weight training sessions and 3 cardio sessions a week without problems.
The suggested target heart rates of between 60-80% of maximum is a pretty wide range and if you gain well you will be more likely to thrive on the high end and if you gain slowly should stay at the low end and work up SLOWLY to the higher end of the scale. There are a zillion target heart rate calculators on line that will apply and calculate the formula for you based on your age and perceived fitness level. Just do a search on google using "target heart rate calculator" as the search words.
If you are a heavier athlete or older, make sure your choice of CV work is compatible with your joints. Sled pulling
Bike riding or swimming are good choices to keep joint stress minimal. And if you are lighter/younger, or have robust joints jogging is a fine option and can be done anywhere without equipment. My favorite is the sled for a variety of reasons.