How to extract certain number LEFT from the criteria if it's found in the cell

I have a very long list of product description from which I need to only extract the capacity of HDD. Basically what I need is a function which would return me the number's left from "GB". Some examples of the the product description:
ESG_HDD_SAS12G_600GB_10K_2_5_CFG
ESG_HDD_SAS12G_ISE_300GB_15K_2_5_CFG,
ESG_HDD_SAS12G_ISE_512E_900GB_15K_2_5_CFG

As you can see the position of "GB" and the associated numbers always changes so the common string function is not working for me. Any help here would be highly appreciated

If that data is in A2 down, then you can use this in B2:

=MID(A2,SEARCH("GB",A2)-3,3)

then copy down.

Hope this helps.

Pete

=trim(right(substitute(left(a1,find("gb",a1)-1),"_",rept(" ",len(a1))),len(a1)))

Thank you both for the reply! Both worked but the function =trim(right(substitute(left(a1,find("gb",a1)-1),"_",rept(" ",len(a1))),len(a1))) is working for me even better as I can use it in IF function combining with other criteria. So amazing that I've got replies in such a short time. This was my first time I posted here something and I feel now encouraged to do it more frequently! cheers