I am in the same boat as we work toward investing in PressSign or similar.
You can use the eXact alone to adhere to GMI standards. The easiest way is to use the job tool. Create a Job template that consists of the GMI L*a*b* targets and TVI targets. Input their respective tolerances. Use the job tool to measure on press. You have access to the entire TVI table in one view (CMYK at 25%, 50%, and 75%). You can tolerance CMYK and RGB overprints. I haven't found a way to do grey balance, as the grey balance is determined based on your K and paper values. That being said, if everything else is in line, your grey balance will pass as well.
One big struggle I am having with GMI is that they base their targets on ISO12647 (2004 amd 2007). This assumes a white point of 94, 0, -2. We are printing on recycled board, where the whitepoint sometimes doesn't even reach 90 on the L scale. When that is the case, it is impossible for us to achieve the target L value of yellow, which is 89. Technically, all the solids and overprints become more difficult to hit when you move from the standard board. GMI also doesn't care what you certify on. We can do our certification on clean virgin board and hit the targets no problem, but then running production on recycled board proves much more difficult. I've inquired about substrate relative targets, but to no avail, likely because it would complicate their end enormously.