ΔE 30.5, loose: different enough that you should pick on purpose. Gladius Bronze is noticeably lighter than Fulgurite Copper.
Can I use Gladius Bronze instead of Fulgurite Copper?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Gladius Bronze is noticeably lighter than Fulgurite Copper.
Can I use Fulgurite Copper instead of Gladius Bronze?
The same difference applies the other way round: Fulgurite Copper is noticeably darker.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.