Holdings of SPDR Gold Trust GLD, the world's largest gold-backed exchange-traded fund, rose 0.7% to 1,035.93 tonnes on Monday, their highest since late-March. By Arundhati Sarkar May 18 (Reuters) - Gold prices firmed on Tuesday after hitting their highest in nearly four months, helped by a weaker U.S. dollar and as investors used the non-yielding bullion as a hedge against rising inflation. "The narrative is clearly shifting towards inflation ... but perhaps more critically, you have got the U.S. dollar weakness, which is probably the key and prime driver," said independent analyst Ross Norman.