There had been a few instances of people discussing swapping gearsets in E92a equipped GM vehicles and being unable to correct rev matching or eliminate errors related to incorrect gear ratios. The tables to correct this behavior exist, but unfortunately were not mapped in the editor aside from one single table that was used for torque multiplication calculations. We located the relevant tables peopled needed and had HP Tuners graciously roll out the addition of these to all E92a operating systems. Update to 5.1.1662 or later for access.
There are two tables labeled Trans Gear Ratios (ECM 20090) and Trans Gear Ratios For Speed Matching (ECM 20265). The first is used for torque calculations while the second is pertinent to the rev matching feature. Set both of these to whatever your new gearset is.
The next tables to edit are Trans Gear Ratios Min/Max (ECM 20261/20262). You can take the easy route here and set the min table to 95% of your new ratios and the max table to 105%, or you can calculate the percentage difference from your original gearset in the tune and apply that difference to the new ratios. The +/- 5% method will be fine, though.
The last table affects clutch slip detection, and this one is important. This table is located under the Transmission -> Diagnostics table and is called Gear N/V Ratio Limit (ECM 20855). For this table, 9th refers to reverse. The factory uses 8% over as the value for second through seventh gear as well as reverse with a much larger value in first gear (your takeoff gear). You can set the first gear ratio to ~50% over whatever your actual gear ratio is and set the rest to 8% over, or you can use the same method of calculating the stock ratio and applying to your new gearset ratios.