I would store just the number. Then you can either remember that it is GB, or put that in the col name. Such as: table_memory_gb or table_memory_mb. It's kind of hard to say without seeing your entire scheme. It could be that your data is not properly normalized.
Regardless, you should never have mixed units in the same field. You should store the data in a fixed format, and the alter how it is displayed (not how it is stored). Hope that makes sense.
This kind of stuff is a pain in the ass with access.
In PHP, you could easily take the number, do a test to see whether it's size would be better displayed in GB or MB, and then display it appropriately. You can still do that with access, but you have to dig into the VB of the report.
