Page "Metric prefix" Paragraph 36

When an SI prefix is affixed to a root word, the prefix carries the stress, while the root drops its stress but retains a full vowel in the syllable that is stressed when the root word stands alone.

For example, gigabyte is, with stress on the first syllable.

However, words in common use outside the scientific community may follow idiosyncratic stress rules.

Kilometre is commonly pronounced, with reduced vowels on both syllables of metre.