If you were spelling it out, you would write two one-gigabyte links, using a hyphen to create a compound modifier. However, a hyphen is not correct when using symbols.
With normal facilities available, don't use any sort of space: 1GB. This is the norm.
With non-lining proportional digits, that can put the 1 very close to the G, so where you can, you may prefer to use a thin space: 1 GB for example by employing an HTML entity
Two 1GB links (no space, normal use)
Two 1 GB links (thin space, might be preferred)
Two 1 GB links (normal space for comparison)
Different display methods may use a thin space which is more- or less-obviously thin.
Section 5.3.3 of The International System of Units states:
The numerical value always precedes the unit, and a space is always used to separate the unit from the number.