For G&G it might not be a bad thing that the GBP drops as I believe their bulk ships outside the UK, meaning their buyers will buy at a lower rate. However, almost all their raw materials are bought from the EU, which will effect their margins. Also, their main bespoke guy Daniel is Swedish, and with Boris and Nigel wanting to throw "immigrants" out it might effect G&Gs bespoke side of the business. We currently don't know which kind of customs will be added to UK goods after the exit, for retailers in the EU. May we live in interesting times!
I don't think you will see any changes in the US retailer prices, but I might be wrong.