Gujaratis are less in the Indian Army for the same reasons that people from many major states were traditionally less represented in the Indian army [some of these are getting amended recently].
Until the revolt of 1857 the Indian army was more of a representative of the general populace. After the revolution that bankrupted the world’s largest corporation, major changes occurred in the army and administration.
One of the changes was the evolution of Martial race and Martial Races, Theory of - where the Raj identified minority groups that are known for the staunch loyalty and with less political involvement. They found the Sikhs, Gorkhas, Kumaonis, Pathans, Garwahlis and a few other groups in India’s edges right for this. The groups chosen were geographically, culturally and politically isolated from the majority population of India. Thus, began a massive transformation of the Indian army.
When India got the independence, the political leaders of the Republic found this arrangement quite useful to sustain the democracy. By making sure that the army was drawn from the minority groups with little political power in the edges of India, they made India coup-proof. Thus, even if the Gorkhas, Sikhs, Garwahlis and Kumaonis of the Indian Army in theory could walk up to the Parliament and take over, there is little way for them to get the legitimacy of the majority & thus to rule. This is in contrast with Pakistan where Punjab controls both the political power and the army.
By some careful planning & chance, it also happened that economic power of India shifted west, administrative power shifted south, military power shifted north, political power was in centre/east. Thus, to get India running a politician from UP would have to work with a civil servant of officer of Kerala, an army officer of Punjab and a businessman from Gujarat. Sort of four locks to keep India into one.
Even though the Indian republic no longer does such discrimination in hiring, through network effects and public belief, majority people are in various regions continue to follow the professions of their regions. The lower unemployment of Gujarat [Indian states ranked by unemployment] also reduces the incentive to join the army.