The club have said consistently that they were concerned about the Gala match purely becasue of the issues when they played at the Swamp recently. To the point that they were deliberatly restricting open ticket sales to make sure that they were not finding their way to large numbers of away fans. So, they won't be bothered abut the low attendance the other night.
The Newcastle semi on the other hand must be a concern to them. We have a real problem now with midweek games generally - as a fan base, we find it harder to make the commitment to Tues/Weds games that we can on weekends - this from a combination of work commitments, parking (walking back to your car at 10pm on a freezing night is much more of an issue than 5pm on a sunny afternoon), the number of blues who live well over an hour away from the ground, and the various restrictions in ticket transfers in place.
So they will have to work out how to make these games more attractive, but short of either adopting the rag approach of bundling them into your ST, or pricing them for nothing (and I'm genuinely not sure that would even work) , I'm struggling to see how they get out of this mess. Safe and efficient traansport to/from the ground bundled with match ticket would be the best way imho.