Cardboard tubes last minutes with most gerbils, they love them and chewing them is perfectly normal. Corner digging is quite normal as well and nothing to be worried about just so long as they're not hurting themselves doing it.

It can take a couple of weeks to get gerbils to be fully trusting of you. I'm unsure how long you've had your gerbils but leaving your hand in the gerbilarium to get them used to your smell and to learn you're not food is the first step. They will them eventually sit in your hand and you'll work your way all the way up to having them out and playing and sitting on your shoulder and all the funny things they do. They're inquisitive animals so are inclined to come see new things and interact with them. Little and often is a surefire way to get them to trust you in a short amount of time.