1. Gary... (ride) his bicycle while Alan was skateboarding.

1. Gary was riding his bicycle while Alan was skateboarding.

В данном предложении нужно использовать Past Continuous, так как описывается действие, которое происходило в определенный момент в прошлом (когда Алан катался на скейтборде).

Ответ: Gary was riding his bicycle while Alan was skateboarding.