Atleast not according to the Boeing web site. The whole 777ER thing is very similar, I think, to what Eastern did with the DC-8s when they were first delivered. Origionally Douglas was to produce DC-8As and DC-8Bs. The As were later the -10 models designed for domestic US service. The Bs were the heavier weight overseas models, later called the DC-8-20. When Eastern took their first DC-8s they called them DC-8A and DC-8B in a similar and familiar fashion to the DC-7A, DC-7B, etc. Delta and a few other airlines protested this. They were origionally allowed to do this because origionally the DC-8s were the A & B model. However, under continued pressure from Delta, Eastern had to change back to the -10, -20 designation.
Now, as for the 777IGW, according to Boeing it is the IGW. There is no ER 777s (atleast, not yet). However, for years people became familiar with the 767-200ER, -300ER and now the -400ER so the term ER sticks out in people's minds more than IGW, besides, it sounds better!
Regards from St Louis!