Gonbad-e Kavus is a long way from anywhere familiar. The nearest airport is Gorgan, about 90 km west, which has daily flights from Tehran. From Gorgan, regular buses and shared taxis reach Gonbad in around 90 minutes. Direct overnight buses also run from Tehran to Gonbad, taking around 8 hours.
If you are coming overland from Mashhad, allow most of a day; the road runs through some of the most underdeveloped parts of Iran but the scenery improves as you climb into the eastern Alborz mountains. The border with Turkmenistan is close, but the only open crossing in this area is at Inche Borun further west.