What is the importance of the initial letters for some of the words - like "Gazing Grain" - in the poem 'Because I could not stop for Death?'
The alliteration in this poem helps to accentuate the feeling of the forward motion of natural beings in the poem. It highlights the mode of transport and helps us to feel we are in there too. It is not a modern car, it is not a train carriage - that has a different rhythm. This rhythm has the forward 'pull' of horses hooves, accentuated by the repetition of some intitial sounds such as 'R' - we can imagine ourselves jolting forward as the horses necks pull away:
'where children strove
At Recess-In the Ring'
...the heavy R sound adds to the alliterative and assonance qualities too.
Another example is:
'For only Gossamer-my Gown-'
...which has an additional galloping sound, quickly following by a tripping one
'My Tippet--Only Tulle'
... in which we imagine a lady being pulled along so quickly that she could trip over her dress or uniform - it emphasises the sense of hurry and time and lack of control ('because I could not stop' - too busy?)
The repetition of the initial consonant sound g in gazing and grain is called alliteration. You also see alliteration in line 7 with the l-sound in labor and leisure and line 15 again with the g-sound in gossamer and gown. Alliteration is a sound device that, often used with assonance (the repetition of vowel sounds in neighboring words), adds flavor to the poem. Assonance is found in the long-a sound in gazing and grain.
Gazing grain is also an example of personification, giving inanimate objects human qualities. Here, the grain is gazing at the sun and/or the passers-by. The grain here also parallels the children in two lines above.