Alright - the reason I am posting this topic is to explain how the portal between worlds might work (according to FMA logic). Feel free to agree or disagree because this is a pivotal part of the movie and essential for the resolution of the series, so it would be great if we could all figure out how the heck it happened.
Before I begin, some review of relevant transmutation circles/information.
1) Grand Arcanum: An ancient method of creating the philosopher's stone apparently discovered by Hohenheim and passed down in the secret alchemic tradition of the Ishbalan people. This is the transmutation array used to create the philosopher's stone in Liore, as well as the one seen in the underground city in Central.
2) Human Transmutation: The circle used by Ed and Al at the beginning of the series when they attempt to resurrect their mother.
3) Gate Array: First seen very late in the series when Dante sends Hohenheim to the other side of the gate. It appears on the stomach of Rose's baby. A similar but more complex symbol is seen in the movie at Professor Haushoffer's villa (pictured below).
4) Complex Gate Array: This is the array which Al makes in the underground city. It can also be seen in the final episode when Al uses the philosopher's stone to resurrect Ed. I'm not really sure of it's distinct purpose at this point. It seems to be used to call forth the gate, but so does the star shaped Gate Array.
5) Eckhart's Alchemy: The symbol for Eckhart's alchemy is an eye, very similar to the eye adorning the Gate of Alchemy. Her method of alchemy seems to revolve around the Gate Children/Homunculi.
6) Alchemy's Source: One of several major revelations of the series; Hohenheim tells Edward that energy cannot be created or destroyed and thus the energy for alchemy finds its source in the souls of those who die on the other side of the gate. We can infer from this, that the only way to reach the gate from the Earth side of the portal is through death. For proof of this fact, look to Episode 50 "Death" when the death of alter Edward allows Ed's soul to reinter the gate and find his real body.
7) Homunculi and Alchemy: The series implies, and the movie confirms, that homunculi ane manifested forms of the gate children. Al remarks: "Of course. The gate always opened when a homunculus was brought to life. Those who came through the Gate can also be material to open it."
8) Why Germany? Because the cycle of alchemy revolves around the exchange of human souls for transmutation energy, it would make sense that the strongest connection between parallel worlds would exist in the places with the most death - and hence, soul exchange. In the manual for the movie, the creators explain that the connect between Lior, Central, and Munich has to do with this very principle. Hohenheim tells Edward "All the lives that have died and will die, cross the gate and become energy." And what is soon to occur in Germany? You guessed it, the Holocaust (not to mention the death from WWI which has already occurred).
Based on the above information, this is my theory of how the gate opened:
The initial attempt to open the gate invovles Envy's body, Hohenheims blood, a human transmutation circle (present as an incantation rather than a drawing), an incomplete gate circle (the star pattern on the floor) and a bunch of German soldiers in suits of armor. Eckhart recites the incantation drawn on a human transmutation circle and touches Envy. Because he can be used for material to open the gate (no matter which side of the gate he is on), this starts a reaction. However, because our world is incapable of actually using alchemy, Eckhart cannot transmute Envy into a gate as Al later does. When Envy attacks Hohenheim, his blood appears to provide enough of a catalyst to open a temporary gate (the emphasis on blood as the lifeforce of an individual is significant, and was long believed to be true). This temporary gate can basically be thought of as a way to achieve premature death. When the soldiers enter it, they are transported along the flow of spirit energy to Armestris. That is why when this disruption in the fabric of reality is created, there are earthquakes at both Liore and Central (the link of spirit energy is strongest at these two places because this is where the most death occured in the FMA world). Thus, we see a Grand Archanum transmutation circle circumscribed with a Gate Array when the ground crumbles in Liore (souls passing through the gate). The unnatural manner in which this occurs (i.e. the soul is transfered allong with the body and mind, instead of the soul alone) turns the soldiers into zombie-like entities. Ultimately, the bodies are taken back through the gate by the forces of alchemy because they do not belong in the FMA world.
The next attempt to open the gate invovles Edward Elric's blood, Envy, and a more complete gate array. Using his knowledge of alchemy, Edward retraces and completes the Gate Array at Eckhart's villa. Envy reprises her role as catalyst, while Edward's blood is the spark of life which gets the reaction going (since the only way to reach the gate is through death on the Earth side, the use of blood is a stimulus for opening the gate). I use the word reaction because I don't believe what has occured thus far can be considered alchemy. It is actually more science in a weird way - performing experiments in an attempt to discover the nature of reality, of how things work. That is why Eckhart's knowledge of alchemy, however imperfect aids her in her attempts to find Shamballa - because alchemy, in this story, is the foundational fabric of reality. Anyway, Edward is able to reopen the temporary portal, and the rejected bodies of the German soldiers fall out.
The final attempt, and the only one to succeed invovles Hohenheim's life, Envy's body, a complete human transmutation circle, the gate array, the complex gate array, and the material of Wrath and Gluttony. Hohenheim must have realized that it requires death to summon the gate. He gives up his life as the price for reaching the gate (through the human trasmutation circle), and Envy's material is expended to keep it open. On the other side, Al uses alchemy to transmute Wrath and Gluttony into a complex gate array. The two meet in the middle, and form a permanent portal to Armestris.