Search Across Columns By Row & Replace Specific Cell Contents

I have created a desk planning spreadsheet in excel. Each week a mailmerge sends out an email to everyone in the office asking them to confirm when they will need a desk in the next week.

As the normal employee doesnt get involved in desk planning I want them simply to state whether they will be in the building or not by entering "GH" (our building) in a cell under each day in the email they get sent.

When the reply comes in we select the cells from the email and then paste them into the desk planning spreadsheet.
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WHAT I NEED TO DO:
Select some cells (must be in the same row - that we have just pasted in).
Press a button on the Worksheet labelled "Auto Assign Desk", which does this:-
For each cell that contains "GH", replace with the right-most cell to the left of the selection that starts "GH_"... (this is the start of a unique desk reference (GH_1_1, GH_1_2,etc.)
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This will assign the employee to the desk they were sat at last, when they are in the office next week.