It's been some time since I played the game, so I may miss something in the review below :)
First and foremost, there are two resources present in the game now - "Oil" and "Raw Materials". Though they are technically infinite (you can't run out of them), you can reach maximum production per turn (i.e. upgrade all your harvesting facilities tp maximum level), thus limiting your production per turn.
It adds more depth to the game and you'll need to consider your logistics more carefully. For example, in AT I usually tried to motorize the bulk of my army but here I can't motorize everyone - the Oil stocks will run dry soon and my armored units will be dead in the water. Air units need even more oil than tanks. And Raw materials seriously limit my production of heavy equipment - arty, tanks, ships etc.
Secondly, the Random Map Generator (RMG) has been improved. You can select type of the map (no water, continent, archipelago etc.) and tinker with different options: quantity of resources, climates, seasons etc. In a nutshell the RMG became more predictable and dependable.
Thirdly, there are several minor improvements to the game. You can have seasons in the game now. AFAIK, there are 3 seasons: summer ("standard season", no penalties), mud season - motorized units can't move beyond roads and have some movement penalties and air units can't fly, and winter - the supply demand of units is doubled.
There are some new units - jeeps, officers and maybe something else (again, it's been some time since I played the game). Jeeps are quite useless as a combat unit but have some limited use as a recon vehicles. Think of them as a toned down Armored Cars with now armor whatsoever. Officers were introduced a couple of days ago (the game is still updated, which is good thing), so you better read patch release notes :)
Every nation has something unique - for example, French have cheap fortresses, AngloSaxons have Rangers (expensive and good infantry), Chinese have Militia (cheap infantry) etc. Not that it completely changes nations' game strategies, but certainly adds some flavor to the game.
Fourthly, the AI calculates its turns much faster. Tens of times faster. From my POV, it deserves a separate mention since I usually play against the AI. Unfortunately, the AI itself wasn't improved, so his intelligence is quite limited.
Lastly, the Product Info says that there are some serious changes to Scenario Editor, but I can't tell about it, as I haven't tinkered with the Editor.
All in all, it's exactly what it says on tin: an improved and enhanced version of AT. If you've got some spare money and some interest in the AT engine, then by all means buy ATG. The game is definitely more complex and more interesting. In fact, the new ATG-only features are so well thought-out, that I can't even think about playing without them.
But if you expect some brand new experience - you won't get it. ATG is not a sequel to AT, it IS AT, but improved.
P.S. Oh, and the new art of units is "hit or miss" - look at the screenshots. For me it's a definite "miss". And I haven't learned to like the art, just to tolerate it, but I cringe every time.
Though there are a lot of different "total overhaul" mods for ATG, I still think that it is best suited for WWI-WWII-Korea period. Its industrial\logistical system represents total wars and if you want to walk some miles in the shoes of WWI-WWII era political and military leaders there's no better game at operational-strategic level.
I repeat myself, but two core characteristics of AT are ability to build your army from a scratch and ability to play any random map. Where the game lacks in technical realism (there are no exact models of tanks, machine guns and whatnot) it more that makes up in the psychological and doctrinal realism.
Thus, I think that its strategic realism is even better than that of WiTE. WiTE suffers from the most well-known wargame disease - players' meta-knowledge of the historical period. For example, good players use it to predict that there will be severe penalties for Axis during winter of 41-42 (but not during other winters) and they PREPARE for it, though the real-world penalties were caused by the LACK of preparation for that winter season. Thus, the strategic uncertainty is eliminated from the game completely.
But I digress and too talkative as usual :) Sorry for that :)