Ek OnKaar Sat Naam
There is no need to be concerned over anybody not visiting a particular gurdwara, however special it is in the hearts of Sikhs. A gurdwara is not like the Kaaba in Mecca that one has to make a pligrimage to, by those of Muslim faith, or the places of pilgrimage of Hindus. Furthermore the US president is not even a Sikh.
Nothing at all is lost, and if one is mourning over a lost photo opportunity then one should reread SGGS ji. If he wishes to visit a Gurdwara, then there are many in the US, and each is equal to Harmandir Sahib in terms of a gate to the Guru. The doors will always be open to those willing to enter humbly.
Sat Sri Akal