What is the defining characteristics of a Gap Species?
The previous answer from @Ark Lomas is true to the extent that gap-loving refers to preference of an open habitat. It was found that these species e.g. have improved flight abilities and are in general geographically more wide-spread.
Here is prove:
Ecology of Tropical Butterflies in Rainforest Gaps
J. K. Hill, K. C. Hamer, J. Tangah and M. Dawood
Oecologia
Vol. 128, No. 2 (Jul., 2001), pp. 294-302

Tropical forest gaps are ephemeral and patchily distributed within
forest areas and have very different light environments compared with
closed-canopy forest. We used fruit-baited traps to investigate if
gaps are exploited by more opportunistic butterfly species compared
with closed-canopy forest. Gaps supported a higher diversity of
butterflies in terms of species evenness but closed-canopy sites
contained species with more restricted geographical distributions.
There was little similarity between the assemblages of butterflies
trapped in the canopy and those in either gap or closed-canopy sites,
but the greater similarity was with gaps, and increased diversity in
gaps was partly due to canopy species turning up in gaps. Dispersal
rates (as measured by recapture rates) were higher in gaps and there
was evidence that butterflies in gaps had relatively larger and
broader thoraxes, indicating a flight morphology adapted for faster
flight. These results support the notion of a distinctive gap fauna
comprising more widespread, mobile species. Habitat modification that
opens up the canopy is likely to result in an increase in these
widespread species and a decline in understorey species with
restricted distributions.