Can regular cradio, endurance cardio I guess you would call it, be just as effective as HIIT? Sometimes it's more tedious to go slow, I rather just run. My goal is to gain weight(muslce) and fat loose.
If I run regulary in the mouring 3 to 5 times a week on an empty stomach for 25 - 30 mins, along with the GVT I'm doing and a good diet, I should just loose fat, and not muscle right?

Right. But if you're bored and want to do HIIT, do it.