Final answer:
Mendeleev was able to predict the properties of elements for the "Gapson Periodic Table" by analyzing the recurring trends and patterns in the properties of known elements and arranging them based on increasing atomic mass.
Step-by-step explanation:
Mendeleev was able to predict the properties of elements that belonged in the "Gapson Periodic Table by looking at the recurring trends or patterns in the properties of known elements. He observed that elements with similar properties appeared at regular intervals when arranging them in order of increasing atomic mass. Using this information, Mendeleev was able to predict the existence and properties of then-unknown elements that would complete the periodic table.