The quick answer, yes. Peanut butter is perfectly safe for gerbils to consume. There is a tremendous amount of misinformation about peanut butter and gerbils on the internet. Gerbils can eat peanuts. Peanuts are a good source of Vitamin E and Vitamin C which are both antioxidants. They also are high in monosaturated fats and can help lower bad cholesterol and raise good cholesterol.
If you are offering a natural peanut butter (ground peanuts) a small amount may be offered a few times a week. Overweight and obese gerbils do not need the extra food. Most peanut butters you buy in the store contain other ingredients. Sugar is often added as well as other agents to prevent the peanut butter from separating. If your peanut butter contains added sugar, only offer it once a week, as the sugar is not good for them.
The easiest way to give your gerbils peanut butter is to smear a tiny dollop of it on the side of the cage with your finger. Gerbils will eagerly lick it off the walls. After an hour, just come by and wipe up any uneaten peanut butter. You do not need to water it down. Gerbils can not get it stuck in their mouth or choke on it. Gerbils are often a bit messy while eating peanut butter so may stop frequently to lick their paws clean.
Some gerbils may benefit from more frequent offerings of peanut butter. Peanut butter is a quick and easy protein and fat boost for breeding females and their pups. Sick gerbils or senior gerbils with teeth problems who are losing weight may benefit from daily offerings of peanut butter.