How to insert vector operators (such as the del operator symbol) in Pages?

Select the last menu item on the Edit menu (Emojis & Symbols). This will open the Characters panel. If you do not have a Math Symbols category as shown in the following capture, then click on the gear icon, select customize, and add the Match Symbols. The Del (nabla) is among them.

Position your insertion point where you want to insert these Vector characters, and then double-click the particular symbol to insert it.