The FTSE 100 made a cautiously positive start, up 0.4% to 6,728.21, as investors await the US interest rate decision later.
The joint venture is expected to realise 'substantial' cost synergies and help GSK scale its consumer healthcare business, the UK pharmaceutical company says.
GSK says it expects the JV to generate total annual cost savings of £0.5bn by 2022 for expected total cash costs of £0.9bn and non-cash charges of £0.3bn.
Planned divestments targeting around £1bn of net proceeds are expected to cover the cash costs of the integration.
Oil rig maintenance specialist Gulf Marine Services' (GMS) full-year results are expected to be broadly in line with guidance, but the company says performance was unlikely to improve next year and that it would breach its debt covenants, sending the shares tumbling 58.2% to 13.6p.
The company said it was experiencing an ongoing underutilisation of its vessels, owing to a number of factors, including an oversupply of vessels across the industry.
Online gambling group 888 gains 6% to 180.2p as it says it remains confident that adjusted earnings for the full year would be in-line with its expectations, citing progress against strategic objectives in the second half of the year.
'The group plans to work collaboratively with and cooperate fully with the FCA in its enquiries,' the company says.
The FCA was probing announcements made by the company between 6 March and 24 October. Yu Group shares tumbled in October when the company warned it would post a loss, partly because it had miscalculated its revenue intake.