Can someone walk me through how using Voice Gateway and/or trunks might be used/what they are for?
You can share a SPn trunk with a 2nd provider via a Voice Gateway. A Voice Gateway can't register to a provider, therefore it can only be used for outbound calls.
GV doesn't work calling some tollfree numbers, so I setup VG1 for a tollfree provider like this:
Name: Tollfree
Access Number: sp2(tollfree.future-nine.com)
DigitMap: (<**>18(00|88|77|66|55|44)xxxxxxx)
AuthUserID: 4801234567
And added the following rule to my Phone Port OutboundCallRoute
{(Mvg1):vg1}
I have it setup when I dial 8005551212 the call goes out SP1 (GV) and 18005551212 is routed out VG1.
Of course, to use the Voicegateways, at least one of the SPx must be setup with a SIP provider or set up as a pseudo-SIP. In other words, you cannot have all your SPx configured for Google Voice. Search the forum for more detail information of Voicegateways (RonR posts).