Gerber files can usually be processed and converted with some kind of dedicated PCB/Gerber software. There is many of various extensions used for gerber files so some specialized formats may use common extensions but still be able to convert GBP to PHO conversion.
Convert Gerber Bottom Paste Mask output format file to Gerber photoplot data.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert gbp to pho on Windows
A PCB and FPGA design software
Convert from gbp file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.