First, backup your current GMail contacts. Make sure a few of them have phone numbers associated with them so that the necessary fields are there. In Gmail, go to contacts then Select Export and choose All and the Google CSV format. Save As google_backup.csv.
Next make sure you have google sync installed on your blackberry, but do not sync yet, or at least not contacts.
Now you have several choices for the contacts on your current phone. Basically you need to get them into spreadsheet format the easiest way possible.
Now open google_backup.csv (using Open Office Calc of course)and Save As google_bb.csv so that changes you make to this file won't screw up your backup. Now cut and paste from your current contact spreadsheet into the relevant fields below your current contacts. Finally, erase the non phone contacts above. It's better to do it in this order so you have something to use as a reference when cutting and pasting. Remember to delete the whole row(s) when getting rid of the contacts you don't want on your phone. Finally, save this file.
Now go back into Gmail contacts, choose all and Delete Contacts (don't worry, you have your backup!). Now choose Import from the upper right and import the google_bb.csv. Now open up google sync on your blackberry and under options choose to sync contacts and then sync now. Viola!
Now uncheck sync contacts if you want and re-import your original contact list. I actually took the opportunity to go through the file and clean it up. Again, leave the original backup file intact!!!!!!! ie DON'T CHANGE THE ORIGINAL BACKUP! This saves you lots of time if you mess up.
The whole process takes about as much time as, or less, than reading this how-to. Let me know how it goes!