The Gospel of Guru Nanak has yet to be taken out of its ecclesiastical and theological atmosphere, to be used as a text-book of the Science and Art of life and worked into the daily life of the whole world. The human spirit it invokes is universal. The English, the Americans, black and white, all must receive it in full measure. Guru Nanak and Guru Gobind Singh are theirs, the Sikhs may adore them as they choose, but they are truly the Prophets of the Future. Men gathering on the banks of the Ganges, the Mississipi, the Thames, and the Seine, will be reborn in the spirit of the Guru, with no past history to recount, but with the new and future Glory of the Nam inspiring them. (Taken from essay - the Sikh Nation, Prof. Puran Singh).
Also, here is what Principal Teja Singh said about it atleast 50 years ago (maybe more). He first describes Guru Nanak as a missionary par excellence:
Then, talking about the prevelant times (mid 1900's) he says that the "so called educated Sikhs", need to realize that it is their duty to propagate their faith (first among the community, then beyond):
Guru Nanak was a missionary in the truest sense of the word. His whole life was a message. He traveled over a greater part of land than any prophet has ever done in the world. And when we consider the difficulties of moving about, the hard times, and the diversity of political, social and religious regions through which he had to pass during his travels. We cannot but marvel at the energy and patience with which he adapted himself to the ever-changing forces of his time.
I would advise those wanting to understand the missionary (ambassadorship) aspect of Sikhi to read Principal Teja Singh's article titled "Guru Nanak and his Mission" available here.
Even among the educated there is a small class of men, whose hearts burn for the advancement of Sikhism, but being hard workers in their professions and finding with difficulty sufficient time to learn about the complicated problem. They can but turn to those for assistance who are set apart and maintained as their theological trustees, the Granthis and preachers. In the general scramble for the Government Service, it goes hard with them to think of self-sacrifice and to make a bold jump for the Guru themselves.Now it is clear that the chief work of missionaries lies with the Granthis and preachers, and for years to come it will have to be done through them alone. Until the educated people also realize that their indifference is fatal to the progress of Sikhism and that the later they come into the field, the harder the task will be against the contending forces