Round up versus Gallup on rushes

Gallup and Roundup are both the same -> it's Glyphosate under a brand name.

Either way I'd be using an MCPA on rushes rather than Glyphosate

Yes it does but you may need to repeat the application. I found the best killout that I got was weed licking/wiping them in both directions then a month or so after they were dead I mowed them with the disc mower. Any new growth that sprouted I weedlicked again when they were tall enough to do so. There was a few bits that grew the following Spring but once I weed licked them they died off. That was about 8 years ago and the area (about a quarter of an acre) has remained clear of them up until this year where I noticed a few sprigs starting to grow. I will tackle them shortly.

Apparently it does as it acts as a sticker.