Unfortunately, DE is not effective as a dewormer... once it is wet it loses its abrasive properties...
Earthworms, snails and slugs are the primary intermediate host for gapeworms, so free ranging actually gives them more exposure to picking up gapes... once overloaded, they show symptoms like panting/gaping/yawning and become sedentary due to obstructing the esophagus so much that it also inhibits their breathing... unless treated within 24-48 hrs max once those symptoms show, they usually don't recover... once some have gotten gapes, the rest will follow... gapes are very, very difficult to eradicate, if not nigh impossible... I highly suggest getting a fecal sample and having a fecal float done from your drake to confirm if possible... if it was gapes, I also suggest a routine deworming regimen with either Safeguard or Valbazen...