ग्रैंड ट्रंक रोड= Grand Trunk Road
जी टी रोड = G.T. Road

This is the name of Grand Trunk Road which is called in Hindi and other Indian languages.It may be the influence of English language.Today this road is a main highway of India and it is called National Highway 2 (NH2) officially in India and N5 National highway in Pakistan.
This road was built by Sher Sah Suri, the great Pathan ruler. No doubt, at that time the English was not present, and this road had another name.

According to Wikipedia,

In the 16th century, a major road running across the Gangetic plain was built afresh by Pashtun emperor Sher Shah Suri, who then ruled much of northern India. His intention was to link together the remote provinces of his vast empire for administrative and military reasons. The Sadak-e-Azam ("great road") as it was then known, is universally recognized as having been the precursor of the Grand Trunk Road. his road was later improved by the British rulers of colonial India. Renamed the "Grand Trunk Road" (sometimes referred to as the "Long Walk"), it was extended to run from Calcutta to Peshawar and thus to span a major portion of British India."
I think it explains the fact.


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Note added at 1 hr (2007-11-19 16:56:12 GMT)
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If you want the name of the road given by Sher Sah Suri, then it will be : सड़क - ए-आज़म। It has no, I repeat , no hindi synonim.