In a small bowl, whisk together the ketchup, rice vinegar, brown sugar, and tamari until smooth. In a separate small bowl, mix the cornstarch with the water to make a slurry. Set both aside.
Season the chicken pieces with salt and pepper. In a shallow bowl, combine the cornstarch and gluten free flour. Dip the chicken into the beaten eggs, then dredge in the cornstarch mixture, pressing lightly to coat.
Heat about 2 inches of vegetable oil in a large deep skillet or Dutch oven to 350 degrees F. Fry the chicken in batches for 4 to 5 minutes, turning occasionally, until golden and crispy all over. Transfer to a paper towel-lined plate.
In a separate large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat. Add the bell pepper pieces and stir fry for 2 to 3 minutes until crisp-tender.
Pour the sweet and sour sauce mixture into the skillet. Add the pineapple chunks and bring to a simmer. Stir in the cornstarch slurry and cook for 1 to 2 minutes until the sauce thickens and turns glossy.
Add the crispy fried chicken to the sauce and toss gently until every piece is coated. Serve over steamed rice with a sprinkle of sesame seeds on top.