First, 8 days may not be enough time to observe a response. I'm not sure, but some other species with foliar treatment require up to14 or more days. A quick search on the internet suggests that a 2% active ingredient with surfactant is a suitable control treatment. If the plants aren't controlled, (give the treatment 21 days to see if it takes), another treatment is possible before the plants begin to flower.
From you question, there isn't enough information to know exactly which formulation of Roundup you are using. In NY, the target species must be listed on the label, or the label must VERY generally stipulate something like "these and other types of brushy species."
Here is a link to the 2(ee) that allows the use of "Roundup Pro" herbicide on garlic mustard. http://pmep.cce.cornell.edu/profiles/herb-growthreg/fatty-alcohol-m... The 2(ee) label http://188.8.131.52/ppds/518499.pdf says to mix at 1 to 3% (1.33 to 4 fluid ounces of Roundup Pro into one gallon of water). Your solution of 2% active ingredient seems to be in the range.
Here is the main label for Roundup Pro http://184.108.40.206/ppds/531333.pdf
Here is the link to search for other labels that might be registered for control of garlic mustard, but I suspect that only Roundup Pro is labeled for use in NY since that has a 2(ee), although a different product (e.g., trichlopyr) might be labeled. http://pims.psur.cornell.edu/
Thank you, Peter.
I agree with Pete - wait and you may find you were successful. In the future you may want to consider an late autumn treatment for garlic mustard. You can treat it then without harming other non-target plants. Also, in the spring the 2nd year garlic mustard plants bolt, flower and produce seed rather quickly, so timing your treatment may be tougher.