Once the keg is clean, and sanitized, fill it with CO2 (you only need 5psi for this case) then pull the relief valve in the lid to remove the pressure. You'll still have CO2 in the lower part of the keg. Fill from the bottom (obviously) and the CO2 will blanket the brew as you fill.
BTW, 'beergas' is a mix of CO2 and nitrogen (typically 75% nitrogen, 25% CO2, but can be 80/20 too) and not something you would carbonate with. You carbonate, typically, with CO2. IF you want to use a stout tap and get the typical Guinness style pour, you would set the keg on beergas once it's fully carbonated. You'll also need to have the beergas at a higher PSI than CO2 in order to maintain the carbonation level. I've not done this (yet) since I really don't have the space for it. It is something I plan on doing this fall/winter though. :rockin:
Something to keep in mind, depending on what you ferment in, you could transfer with a CO2 push too. Carboys can be fitted with the caps that allow you to put the racking cane (in the middle) and a hose to the CO2 regulator on the other nipple. You only use a couple of PSI to get the beer to flow. It does make it easier, IMO/IME this way. I also use the liquid post on the keg to fill it. So, I don't put anything into my sanitized keg other than beer, when filling it. Since I fill two 3 gallon kegs with every batch, I do watch as it fills (covering the opening with a clean, sanitized, towel).