Gratuitous inducer is a molecule which is structurally similar to another molecule that induces transcription for a specific product, and which can also initiate transcription for that product when the official inducer is absent.

Function with example-

The lac operon introduced by jacob and monod, is the classical model for gene regulation.  When lactose is present the repressor dissociates from the operator and the genes are transcribed. Control of lac operon does not only depend on association or disassociation of repressor, cyclic AMP also plays a regulatory role.  This exihibit will show three dimensional structures of intact lac repressor, lac repressor bound to gratuitous inducer IPTG, and lac repressor complexed with 21 bp symmetric operator DNA.  These structures show the conformation of lac operon in both induced and repressed states, will give information about the lac repressor. 

It is different from others as it is a natural inducer that is capable of inducing an operon while not serving as a substrate for the enzymes being induced.