Those are just the name of the manufacturers. The actual prefixes of the cards currently for Nvidia are GT, GTX and for AMD are just HD. Anything else is irrelevant besides the actualy GPU Name, which would be 7770 in this case. So, while those cards are named as such, they are all HD 7770's.
Also: You CAN Crossfire two of the same GPU model (7770) from different manufacturers. As long as it is a type of 7770 it is compatible.
Here's a chart you might want to look at:
http://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg
Lastly: If you plan on crossfire 7770's I would just spend more and get a single 7870 or 7950 (or an equivalent Nvidia card). Less of a hassle with just one card.