I would interpret what they have said as you would be roaming on Orange towers.
Which may incur roaming data charges, I'm not sure.
But if you have a T-mobile tower next to you, it will always use that over an Orange tower. You can't force it to use the Orange tower.
But yes, GPRS is 2G and EDGE is effectively 2.5G.
"That's it then. Cancel the kitchen scraps for lepers and orphans, no more merciful beheadings... and call off Christmas."