Employees of the British drugmaker went on strike last month, having turned down the company's "significantly below inflation rate pay offer" of a 6% increase and a one-off lump sum of 1,300 pounds ($1,619.41), Reuters reported.
The first strike this month will be held on June 9, Unite said, at all six GSK sites - Barnard Castle, Irvine, Montrose, Ware, Worthing and Ulverston.
Elevated levels of inflation have outstripped wage growth for most British workers, and labor unions representing workers across sectors - from nurses to transport staff - have been on strikes for months now demanding pay increases.
GSK did not immediately respond to a Reuters request for comment.