John Shepheard, country director for UK and Ireland, said there had been "a noticeable drop that is directly attributable to the tragic death ... it's a very unusual circumstance."
On Saturday, Gateway will close its Covent Garden, London showroom as a mark of respect for Diana's funeral procession, although most shops in the capital will open at 2pm.
"We're right in the heart of London and Saturday is our busiest day but we looked at it as a mark of respect not to open," Shepheard explained.
Gateway earlier this week warned it expects profits to be hurt by a recent US strike at United Parcel service (UPS), its chosen courier. Shepheard said he expects other direct sellers to be hurt by the strike. "If they're using UPS they'll hurt ... and they're all using UPS."
Separately, Microsoft cancelled a press drinks party scheduled for last night in deference to Diana's death.