With GDV the stomach can quickly become severely distended. Typical signs are restlessness, drooling, a swollen or tense belly and repeated attempts to vomit without anything coming up. The dog can also become weak, get pale gums and collapse as its circulation is affected.
Large, deep-chested dogs are at higher risk, but any dog with matching symptoms needs to be assessed based on the situation. The diagnosis is usually confirmed with X-rays after initial stabilisation. Treatment means relieving the pressure in the stomach, correcting the effects on circulation and, in most cases, operating to return the stomach to its normal position and reduce the risk of it twisting again.
This is not a condition to wait and see about at home. If your dog has a rapidly swelling belly, is very restless or keeps trying to vomit without result, go to a vet immediately.