This will probably need to be updated soon. There are rumors NIF recently achieved a gain of ~4.4 and ~10% fuel burn up. Being able to ignite more fuel is notable in and of itself.
In the context implied above it is the ratio of fusion energy released to laser energy on target or the laser energy crossing the vacuum vessel boundary (they are the same in this case). So it would have been more precise to say "target gain" or "scientific gain".