There is a really long thread at Google Webmaster Help on the topic of the .GB.COM TLD. Some are of the belief that Google is penalizing or banning these domains.

Of course, to you and me, that sounds ridiculous. There are thousands of .gb.com domains indexed in Google. But these webmasters are complaining that they are not ranking well.

One said:

The thread is pretty heated right now. No Google response has been made as of yet. Of course, when it comes to topics like this, you have to be very skeptical.

Forum discussion at Google Webmaster Help.

Update: Read the thread, it appears that gb.com are not real TLDs or even ccTLDs, which might be the reason behind this.