Are there any mainlands close to Guam?

No, there are no continents or mainlands close to Guam. Guam is the largest and most populous island in the Mariana Islands archipelago, which is located in the western Pacific Ocean, about 2,000 miles (3,200 kilometers) east of the Philippines and 1,500 miles (2,400 kilometers) northeast of Papua New Guinea. The nearest major island group is the Commonwealth of the Northern Mariana Islands, which is about 120 miles (190 kilometers) north of Guam.