I'm not seeing either Charles or Gabrielle in the 1901 in Scotland. There is a possible Charles in Glasgow, but he is several years too young:
1901 DUPREE CHARLES M 17 ANDERSTON GLASGOW CITY/LANARK
(I'd expect your Charles to be aged about 24/25 given that he said he was 29 in 1905)
The one above is living as a lodger with a family called McKay & another lodger Mary Dupree, aged 50, a widow, a sackmaker, born Ireland (his mother?). He was born Glasgow and is a foundry labourer. None of this fits your guy, so I'm pretty sure this is NOT him.
I'm not seeing anything obvious on the English 1901 either.
I wonder if this implies that they came to UK between 1901-5? Or that they are there somewhere, but I'm just not finding them....

Best wishes & a bientôt! (hope the Français is coming along nicely!)
Lesley