Remove the seeds from the pumpkin pulp. Place them in a large bowl of water and swish them around. The seeds will float to the top, making it easy to scoop them out while leaving the heavy pulp at the bottom. Rinse them one last time in a colander.
Spread the cleaned seeds out on a clean kitchen towel or paper towels. Pat them dry as much as possible. For the best crunch, let them air dry for at least 30 minutes. If the seeds are wet when they go into the oven, they will steam instead of roast.
Preheat your oven to 300°F (150°C). In a medium bowl, toss the dry pumpkin seeds with the melted butter or olive oil. Add the salt, garlic powder, and pepper. Stir well until every seed is evenly coated with the oil and spices.
Spread the seeds in a single layer on a large baking sheet lined with parchment paper. Place them in the oven and roast for about 45 minutes. Make sure to stir them every 15 minutes so they brown evenly and do not burn.
Once the seeds are golden brown and fragrant, remove them from the oven. Let them cool completely on the baking sheet. They will continue to crisp up as they cool down to room temperature.