Cassidy was seeking to follow up his third place from the opening London race, in which he was promoted from fourth after Nyck de Vries was handed a penalty for weaving in front of him in defence.
Instead, the New Zealander was dealt damage to his suspension and his rear-left tyre in an early tangle with Gunther - and under the safety car was seen driving close to the pitwall for his team to assess the damage.
This eventually forced Envision to pull Cassidy's car into the garage, resulting in retirement after he had been running seventh in the opening laps.
"Gunther decided to hit me seven times within the first seven corners," Cassidy told Autosport.
"It broke my suspension and broke my left rear tyre.
"Basically, I had a slow puncture. So when the safety car happened I said, 'Well I have a slow puncture, I'm down to this pressure'.
"I was watching it go down, but when you're driving so slow you don't know if it's true or not, and I was asking the team to check visually.
"But then if you box in London from P6 you go to last, you don't recover, and we had to take the risk.
"We stayed out, hoped it was a sensor - and it was clear it wasn't and I had a flat tyre."
Cassidy felt that he once again had the pace for podium contention, and although he was not able to unlock the pace to progress beyond the quarter-finals in the duels, believed he would have been able to work his way through the pack.
He had headed free practice earlier in the day, beating DS Techeetah's Antonio Felix da Costa to top spot by 0.04s.
"Yeah, it sucks," he said. "I stayed out of trouble on lap 1, I did everything perfect. I had a really fast car, and we could repeat yesterday for sure.
"We know from [Saturday's] pace and practice that we were fighting for the podium - and to be taken out by 'Mr Desperados' absolutely sucks."
Guenther refuted Cassidy's comments, stating that there was only one instance of contact between the two - when Guenther was pushed into him by another car.
"I'm very surprised by the comments from Nick, because from my side there was no contact other than the one contact we had when I got pushed from behind by other cars," he said.
Having chalked up an eighth place on the previous day, Gunther's eventful Sunday race continued when he was handed a 10-second penalty for a nosecone change that triggered a pitstop infringement.
A Nissan mechanic was still attaching the new nose when the ready-to-move light on Gunther's car showed up, resulting in the penalty.
It ultimately made little difference, as the German was classified last and 43s down on 14th-placed Oliver Turvey.
