I think they are at least decent backup options, and that Gallagher would have done better than McTominay in the last two Scotland games. He certainly did OK in the appearances he's made already.
I also think Gallagher has been at least as good as McKenna in the past 12 months.
We're not overburdened with choices in defence at the international level, however, especially with the current injuries. So I have no issues with either one of our players being selected. It also reflects well on the club to have someone like Dec get international recognition only after joining us.
If you want to go on recent form, Robertson shouldn't even be in the squad tonight, but I imagine he'll be captain and play OK.