With regard to a wireless USB dongle, what do "Mbps" and "High gain" refur to please?

[Southpaw]

Mbps means Mega bits per second, MBps means Mega bytes per second and a byte is equal to 8 bits.
Wireless dongle to router speeds, and in fact the internet speed from your router to the local exchange are always measured in Megabits, not Megabytes per second.
Not that you want to worry much about that.
What you need to know if buying a wireless dongle is the suffix letter associated with it, which has progressed via B - G - N over the years and there is a newer one coming out whose suffix letter escapes me, but might be CA or AC.
The newer ones are all backwardly compatible with older versions, so all will work with each other after a fashion, but the latest suffix N has a faster rate and more range;
http://compnetworking.about.com/cs/wirel...andard.htm
So yes, 300 Megabits per second is twice as fast as 150 Megabits per second, it is the maximum speed in theory, but usually not achieved in practice.
High gain refers to the ariel that the dongle uses and means that in the opinion of the manufacturer it is more powerful than other similar run of the mill devices.
Might even be true.
The one certain thing about connecting a computer to a router using a wireless dongle is that it will never be as fast as an Ethernet cable connection, mainly due to radio interference from nearby electrical installations slowing it down, and obstruction of its radio signals by such things as walls.
Regards, Bob.