。
The cowboys rounded up the herd of cattle.
Literal
Cowboys [topic-は] cow [attributive-の] herd [object-を] drove-gathered.
駆り集める is a compound verb: 駆る ('to drive, chase, urge on') + 集める ('to gather, collect'). Together it means 'to round up, herd together by driving.' 牛の群れ ('herd of cows') uses の attributively to link 牛 and 群れ. A compact but vivid verb choice for cowboy action.