I've given it to mine before, when I ran out of food, with no problems. I think it's one of those things that you can give, as a treat, but it's not the best for them. Though lots of brands of guinea pig food have wheat in them, so I'm not sure . . .
I'd avoid it, and give them veggies instead as treats
One of my pigs has half a weetabix for breakfast every morning (not with milk and sugar!) She was very poorly a while ago, and I tempted her to eat with this, so now she has it regularly. I'm not sure long term how good this is, but now she won't shut up til she gets it. xx
My girls have had various plain cereals including weetabix. Cheerios make good treats. I would always go for a decent pellet and fresh veggies as their main food, but as long as a food is vegan I will consider it as possible - Rose had tiny squares of white bread last week after antibiotics - I'd had a piece to have witha cup of soup and she sniffed hopefully. I let her have a little, and it didn't do her any harm. I've also given my girls rich tea biscuits and Ginger biscuits (dairy and egg free).
You could also try oats as a treat, i have been told by a rodentologist that she feeds oats and wheat bran to her pigs and they love it. You can get them both from Holland and Barret's for about Â£1 per bag. I feed my pigs a tablespoon every-other day and they hoover it up!