There is a widely adopted introduction process for the gentle leader that is taught by animal behaviorists. How well your dog accepts a gentle leader depends on how you introduce it to him, so just be aware that if you did not provide any de-sensitization period and immediately snapped the leash on then the process could take longer and will require more patience.
I will try to summarize here, but just know there is much more written guidance to this process:
The halter should be introduced gradually and associated with something positive (yummy treats). He will need time to adjust to having something around his muzzle since this is extremely foreign and a significant sign of control by you.
Begin in your house or yard where there are no noises or activity to distract him.
Day 1:
A half dozen times or so, hold the nose loop open with a treat behind it so your dog has to stick his nose through the loop to get it. Use a cue marker such as click, or "yes". Repeat this exercise three or four times during the day, for a dozen reptetitions. Use a cue marker each time you begin (like "halter") to teach him a phrase that means putting the halter on.
Day 2:
Follow the same procedure as day 1, except now when the dog has his nose in the loop, pull very gently on the halter to put a little pressure on the nose loop. Wait a little longer each time before you click/yes and reward. Repeat 5-6 times throughout the day. Do not reward if he struggles or paws the halter.
Day 3:
Continue the same process, except after the dog has done the nose in loop two or three times, snap the buckle closed behind his ears. Let him wear it for several seconds while feeding him treats and keeping him in a positive state. Unbuckle it as long as he is calm. Don't teach him that struggling or resisting results in the halter being removed.
Day 4:
Put the halter on following the same process. Let him walk around the room, using reward markers and treats. When he is calm, attach the leash to the ring under his chin. Try walking him a few yards. Use gentle pressure only. ALWAYS ensure the leash goes from under his chin directly to you, never behind his head or over his neck. Any time he gives to the pressure of turning to you, use your reward cue and treat. Also, click/or cue lots of times when he is near you and not pulling to teach him that not pulling gets rewarded. Remove when he is calm, and not resisting.
Day 5:
Begin the same as day 4. If your dog is accepting the halter and leash well indoors, go outdoors in a controlled environment like your backyard. If you started outdoors, expand the area. As a safeguard, attach his leash to a regular collar so if he slips out or is going to hit the end of the leash hard, you can use the second leash as an emergency stop, and regain control with the halter. This avoids damage to his spine.
Day 6:
Graduate to walking up and down your street. Be prepared for his anxiety level to increase as the environment expands. As always, reinforce with reward cues and treats.
If your dog balks at any point, slow down and back up a step for an extra two or three days. It is normal to occasionally rub the halter against you or paw at it - if he does - then distract him by moving forward, talking to him, and rewarding with forward movement. Reward when he is not resisting.
Day 7:
If your dog has progressed well, you are ready to tackle the world now. Avoid moving any faster than this as you will cause aversion to the halter and situation with you making it longer to address and build a positive relationship.
For those who make the mistake of putting it on their dog and going right out, they can require up to 14 days to adjust with this process.
Hope that helps!
