Gurkhas are not Sikhs, the Gurkhas are a military organization, Sikh is a religious group. Not to say that a Sikh can’t be a Gurkha.
Both Sikhs and Gurkhas have reputation with knives.
Both are respected for honesty and respectability.
Sikhs are experienced at fighting Muslims.
Basically they are Indians, just as most Sikhs ethnically are -- the indian continent is vast and diverse
Sikhs are mostly ethnic Punjabis from the state of Punjab. They are a religious group, so not all Punjabis are Sikhs, but most Sikhs are Punjabis. Gurkhas tend to be Hindus, but hinduism is also am umbrella term for a number of different religions
The language of both is Indo-European and both have a martial culture and both were highly respected from the 1700s onwards as a military group.
The Gurkhas helped the Nepalese kingdom form in the 17th century while the Sikhs who were initially peaceful became militarized as a response to Moslem atrocities in the 1700s and formed their Sikh kingdom in the 18th century -- this fell to the British by 1810 and then they helped the British conquer the rest of India, which the Brits did (mostly by diplomacy) by the 1840s.