Can I use Inteleon VMAX's "Double Gunner" Ability if my opponent only has 1 Benched Pokemon?
Yes you can, but not if they don't have any Benched Pokemon.
Source: Fusion Strike FAQ; TPCi Rules Team (2021-11-11)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH8_79
SWSH8_79