Can’t help on the value, but I can give you a bit more information on the case. The ‘GS’ mark is for George Stockwell, a well-known Birmingham importer of foreign watches. He’s listed in the Birmingham directory of 1912 as “Stockwell & Co. Ltd. - agents to Messageries Nationales Express and Messageries Anglo-Suisse, continental, foreign and general shipping agents, special tariff for small consignments abroad.”
The case was imported from Switzerland and re-assayed in London as .375 gold (ie 9 Karat) and has the date letter (f) for 1921. The odd U-shaped mark is an upside-down omega, which was London’s import mark, in an oval for silver and in a rectangle with clipped corners (like yours) for gold.