If there is always a blank between number and text, You can separte it using the 'position' function and the 'middle' function, Otherwise You could get the data using the 'filter' function
"Is there a similar function to return just the letters?"
You could use the Filter ( ) function, something like:
Filter ( YourField ; "BGT" )
So many ways to solve this problem. (Here is another):
MixedField = "50.00 GB"
MyDigits = LeftWords ( MixedField ; 1 )
MyLetters = RightWords ( MixedField ; 1)
And just to join the party, GetAsNumber ( mixedField ) will also return the numeric portion of the text and number field.