This document from 1926 indeed mentions the gold embargo as detrimental to Soviet-British trade, thus the embargo did exist, at least for some states. On the other hand, it also mentions that in the years its authors investigated (1917 -1925) Britain imported over 6 million pounds of gold and silver from USSR (while exporting about 7 million), so we can safely assume that by 1925 the embargo was nonexistant.
But by 1928 the gold reserve of USSR was at a historical low - while the exact amount was undisclosed, it is estimated that in 1918 it amounted to ~1000 tons of gold, and by 1928 it was down to ~150 tons. Stalin's government took radical steps towards increasing the production, started actively confiscating gold and foreign currency, and cut down on imports (until 1941 Stalin was very reluctant to spend any gold from the reserves at all), but the reserve only started growing again around 1934 - for example in 1932 Soviet mining efforts yielded only 32 tons of gold. (Source: Л.В. Сапоговская, "Золотопромышленность Республики Советов - СССР - РФ: эволюция отрасли в альтернативных системах хозяйствования", "Экономическая история. Ежегодник. 2003" (М.: РОССПЭН, 2004. С. 266-308))
So, while USSR was not prohibited to export gold, there was not much of it that it actually could or wanted to sell, and while grain was not the only export, it was the main source of income in Soviet international trade.