Teaching a gundog a stop whistle command is an important aspect of their training, especially for activities like hunting or retrieving. Here's a step-by-step guide to help you teach a gundog the stop whistle command:

Note: This training requires patience, consistency, and positive reinforcement. Make sure you have a strong bond with your dog and have established basic obedience commands before starting this advanced training.#

Choose the Right Whistle: Select a distinct and consistent whistle sound that will serve as the stop command. This sound should be different from other commands and noises used during training.

Preparation: Begin training in a quiet, distraction-free environment where your dog can focus on you. Have plenty of small, tasty treats on hand.

Associate the Whistle with Positive Experiences: Before using the whistle as a command, associate it with positive things. Blow the whistle and immediately offer your dog a treat or engage in play. Repeat this several times so your dog starts to associate the whistle with something enjoyable.

Leash Work: Start training with your dog on a leash to have better control. Walk together and blow the stop whistle while gently pulling the leash to stop your dog's movement. As soon as your dog stops, praise it enthusiastically and reward with praise, or a fun retrieve. 

Add Distance: Once your dog is consistently stopping when you blow the whistle on the leash, gradually increase the distance between you and your dog. Use a long training lead to allow your dog more freedom while ensuring you have control.

Gradually Increase Distractions: As your dog becomes more reliable in responding to the stop whistle in controlled environments, start introducing mild distractions. This could include other people, dogs, or mild environmental stimuli.

Generalization: Practice the stop whistle command in various locations and situations to generalize the behavior. Use the whistle during play sessions, walks, and eventually during actual hunting or retrieving activities.

Consistency and Patience: Consistency is key. Always use the same whistle sound and command. Be patient and avoid scolding if your dog doesn't respond perfectly. Instead, focus on rewarding correct behavior.

Reinforcement: Gradually reduce the frequency of treats, but continue to reward with praise and occasional treats to maintain the behavior.

Advanced Training: Once your dog has mastered the basics, you can work on refining the command for greater distance and reliability. You might also want to consider professional gundog training if you're preparing your dog for specific hunting or fieldwork.

Remember that every dog learns at its own pace, so be patient and tailor your training approach to your dog's individual needs and abilities.