There's a link below that gives details of Gowlands lotion.
http://hibiscus-sinensis.com/regency/skincare.htmIn the book, it is the horrible Mrs Clay who is advised by Sir Walter Eliot to use Gowlands Lotion for her freckles. It seems to have been a dangerous concotion.
The Times of 31 August 1790 carries a long advertisement for Gowlands Lotion, sold by Thomas Vincent, 'the real proprietor and successor to Dr Gowland'. This suggests that it was not so much a beauty treatment as a medication - 'Gowland's Wash or Lotion was said to be
'an effectual remedy for all diseases of the skin, whether from scurvy, a surfeit, or any other cause however violent or disfiguring'.