So you kept a 5dB boost at the limiter stage in TGP, then sent that to Ozone for more limiting. Depending on your input level to TGP, you may have heavy multiband compression going on, then limiting, then more limiting, which is likely why that one sounds flatter. If you're using Ozone with mastering assistant, that is a different use case than what TGP was designed for. And if you're just wanting an "auto-mastering" type of thing at the very end of a mix, Ozone is the more fool-proof option.
As for how I'd use TGP, it's designed to be used as follows... Insert it when you begin your mix. As you mix, keep an eye on your gain staging and how you're hitting the meters in TGP. The input meters should regularly light the green box on the left. That's the input target. There are three more green target boxes on the gain reduction section for low, mid, and high bands. The low and mid should light regularly on transients and louder elements, and the high should rarely light up. If it lights a lot, that's an indication you're mixing your high end too hot. To the right of this section is the limiter section, and you'll see limiter gain reduction there. If it's doing too much gain reduction or you're going to use a different limiter, turn the knob at the bottom counterclockwise to decrease its level or just turn it off with the power button at the top of that section. As you get your levels in the proper ballpark for the plug-in, you can experiment with how small input adjustments affect your mix, as you'll have a range of level where you'll still hit your targets, so that means there's room to play with how hard you're hitting the multiband compression and limiting, for instance. A little 0.5dB input adjustment may be all it takes to really find a sweet spot.
Even though it's a simple plug-in on the surface, you still really need to understand how it works to use it to its full potential (or to just not mess up your mix).