The item's description reads
While you wear both of these steel gauntlets, any non- magical weapon you grasp with either gauntlet is treated as a magic weapon. As a bonus action, you can use the gauntlets to cause magical flames to envelop one or two melee weapons in your grasp. Each flaming weapon deals an extra 1d6 fire damage on a hit. The flames last until you sheath or let go of either weapon. Once used, this property can't be used again until the next dawn. Source: BGDIA, page 223
Now the first part is clear that it makes non-magic weapons magic, but does the second part using a bonus action to add 1d6 fire damage apply only if the weapons are non-magic to start? I.E. If I wield a +1 short sword would I roll 1d6 slashing +1d6 fire +1 for damage or just 1d6 slashing +1?