All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no doubt that using gold letters falls under the verse (what means): {…And be not excessive. Indeed, He does not like those who commit excess.} [Quran 6:141] Allaah, the Exalted, also ways (what means): {…and do not spend wastefully. Indeed, the wasteful are brothers of the devils, and ever has Satan been to his Lord ungrateful.} [Quran 17:26-27] The case mentioned in the question might also fall under the prohibition stated in the following hadith: the Prophet, sallallaahu ‘alayhi wa sallam, said: "Wearing silk and gold has been made lawful for the females and unlawful for the males of my community." [Ahmad, Abu Daawood, and others] This indicates that it is impermissible for Muslim men to wear gold ornaments. However, scholars considered that the use of gold, of all kinds, is forbidden as well. Besides jewelry, jurists also forbid men to use a gold pen, for example, and all gold-made items.
Allaah Knows best.