Garchen Rinpoche was influenced by his root guru, the Nyingmapa Khenpo Munsel a great deal, of course. However, that's not the only flavor of Dzogchen teachings he received, by any stretch of the imagination (not that you said or implied that).
In the Drikung Kagyu tradition in particular, there is a whole cycle of Dzogchen terma cycle called the Yangzab which was first revealed by the 17th Drikung throneholder, Gyalwa Rinchen Phuntsok, and later was further revealed by the likes of Lho Nuden Dorje and others after him. Thus there is a whole corpus of Drikung Dzogchen for him to draw on as well.
In addition, Garchen has received countless teachings from the cycles of Ratna Lingpa, Chokgyur Lingpa, etc. And almost every Drikungpa I've come in contact with holds the Lonchen Nyingthik, too. So obviously Gar Rinpoche has received Dzogchen from many, many streams, and these have influenced his teaching style, no doubt. Dzogchen is certainly not foreign to him as a practice, whereas with some Kagyupas it could be. He is a Dzogchen master as much as a Mahamudra master, if not more so.
As to how he compares with other Dzogchen teachers, my ability to comment is limited, as I only have one strictly Nyingmapa teacher from the Palyul lineage. Except for the Drikung cycle of Dzogchen, I can say that Garchen's situation regarding having received the teachings of several tertons is not uncommon for Nyingmapas, either. My own lama, Khenchen Tsewang Gyatso Rinpoche, can just as easily teach from Dudjom Rinpoche's texts and the Longchen Nyingthik, since there is a lot of cross-pollination and the various lineages didn't want to miss out on teachings which were universally acknowledged to be of great merit.
Basically, each lineage is going to have it's specializations or cycles particular to it, and the combination of each lama's training, practice, and realization will lead to their own unique blend, too. I think it's pretty safe to say that you'd be hard pressed to have a Drikungpa teach on the Namchö cycle as well as a Palyulpa could, and conversely a Palyulpa would likely not do as good a job in teaching the Yangzab as a Drikung Dzogchenpa would. Then again, there are some exceptional beings who simply seem to have no limit to their scope of knowledge (and Garchen Rinpoche has been known to pull from the Namchö from time to time).
I have yet to hear anything contradictory between Garchen Rinpoche and Khen Rinpoche, and many of the analogies they use are tried and true. I will say, though, that Kyabjé Garchen has a much more streamlined approach to explaining samaya than many other lamas, distilling it all down to the root of love and patience (similar in style to HH the Dalai Lama's public talks), and not having his students worry about the branch samayas. Then again, samaya aren't needed if one realizes the natural state, so that's really not Dzogchen.
Alas, I've not received Trekchöd or Thögal teachings from anyone, so it's beyond my purview to speculate on any substantive differences in how Garchen teaches Dzogchen as compared to other Dzogchen masters. Perhaps someone who's received such instructions from the likes of Kyabjé Dilgo Khyentse, Tulku Urgyen, Chögyal Namkhai Norbu, or another great teacher can lend you some assistance in that regard...
Here's hoping my post was of some benefit, and not just an exercise in me running my mouth