In a large bowl, whisk garlic, pineapple juice, ketchup, soy sauce, brown sugar, and ginger until combined.
Place chicken in a large resealable plastic bag. Pour in marinade. Refrigerate at least 2 hours and up to overnight.
Prepare a grill for high heat; preheat 5 minutes (or heat a grill pan over high heat). Clean and oil grates.
Grill chicken, basting with marinade and flipping occasionally, until charred and an instant-read thermometer inserted into thickest part registers 165°, about 8 minutes per side. Transfer to a plate.
In a medium bowl, toss pineapple with oil. Grill pineapple, flipping occasionally, until charred, about 2 minutes per side.
Transfer chicken and pineapples to a platter. Top with scallions.
