Do you have toys/tunnels/hay etc. for the gp to play/explore/eat?
Noticed when I initially got my male, and he was in his 1st (store) cage, that on occasion, when scared/stressed he would run to the corner, try and climb, and bite at the bars out frustration. Other times, if either of my male or female are outside of the respective cage (her more than him) they will do the same thing, as they WANT back in. If they are doing the sniffing through bars at each other, they'll nibble at the bars too.
With the exception of the sniffing through bars, adding a ton of tunnels to his cage and avoiding having him near/in while doing a bedding change, stopped that. Sometimes, she'll nibble at the bars if I'm not quick enough with a treat, or am getting several veggies out at once, cutting them up, etc. though usually giving one by hand will stop it as the rest is prepared. When on a normal day left on their own, I've never seen them nibble at the bars just to do it. If they have a hiding house, it should stop it too.
Being frozen (freeze or flight) is just because it isn't used to you/environment yet. Treats will break that. Put them in the dish 1st, then over time try hand feeding. I've had luck with a young male and female, by using my hand to "walk" around them; left handed so: index finger is right front leg and thumb right rear, ring is left front and pinky is left rear, while middle finger is head/snout (think of an aardvark). Hand in cage, would = FREEZE or run. Hand imitating a fugly looking tick rodent walking = completely disarmed. I still do this from time to time, if either are getting jumpy for whatever reason. A great time to try it is during fresh grass additions. I'll "walk" the hand over, poke the grass with middle finger tip, then move backwards a few inches. Then repeat several times. If guinea pig is disarmed, it'll eat. Can slowly put middle finger tip up to their nostrils (close w/o touching) then back it away and go back to grass. Male was eating from hand within a couple days after getting him, and female was the same. He was 6 weeks and she was 4 weeks, roughly