In all function generators I know about, both square waves and triangle waves have a default DC Offset of 0 Volts. (They go above and below 0V equally.) Adding in a DC offset is usually a separate setting.
Not HP equipment but related: I recently gave a talk to a local ham radio club about 'scopes and spectrum analyzers using a and its . It is a great "electronics lab on a board" for experimenting and demonstrations. The following screenshots are from the talk, showing an audio spectrum analyzer image of a 1 kHz square wave and how that square wave looks on a scope after only 4 component sine waves are added together. (During the talk these are live images that update real-time as settings are changed.)
Rick - WA6NDR