I need help figuring out the meter of the poem God's Grandeur.

Overall, the poem is in iambic pentameter, but there are some irregularities in the first five lines.

An iamb is a poetic foot consisting of an unstressed syllable followed by a stressed one. In iambic pentameter, each line has five iambs in a row. This is a very natural meter for English.

Most of the poem "God's Grandeur" can be scanned in iambic pentameter:

And though the last lights off the black West went,

Oh, mor-ning, at the brown brink east-ward, springs --

Read the the bold syllables as stressed and these lines will fall naturally into iambic pentameter.

The first few lines of the poem do not work this way. The first line contains two iambs followed by two anapests. (An anapest is a foot with two unstressed syllables followed by one stressed one.)

The world is charged with the gran-deur of God.

Anapests usually give a line a jaunty feel, as if it is moving along quickly. In this case, it gives the poem a conversational feel and gives us the sense that the poet is charging headlong into the poem, eager to tell us about God's grandeur.

Line two can be read to contains five iambs, but line three departs from iambic pentameter again, this time with six iambs. The extra beat in line three gives the line a statelier feel which matches the idea of something "gathering to a greatness."

It gath-ers to a great-ness, like the ooze of oil

Line four has five feet, four of which are iambs, but the first one ("Crushed. Why") is a spondee, a foot in which both syllables are stressed. The spondee brings the reader up short, preparing us for the cacophony that comes in the rest of the line as well as the frustration it is expressing.

Line five begins with the awkward word "generations." This forces the line to be scanned as beginning with two anapests, followed by two iambs:

Ge-ne-ra-tions have trod, have trod, have trod,

We can almost hear treading feet in the line (slowing down with weariness toward the end, as they move from anapests to iambs), and the fact that the line has only four feet instead of five creates a sense of urgency and incompleteness.

For definitions of the different kinds of poetic feet, see the link below.