I would never attempt or recommend using table import for Inventory Items. GP has the ability to create a macro and to play a macro.
There is a process where a user can use the 'mail merge' process to create a very long macro that replicates entering items - I have an old document that I would be happy to share with you. It's a step by step guide to creating a macro - then putting the macro into Microsoft Word - merge the macro with an excel listing of item numbers and finally finishing with a macro that you will 'play' with your cursor in the item maintenance screen - the macro emulates a user entering items at record speeds (pretty fast - much faster than any human can go). Since you are using the interface, GP is hitting all tables correctly.
If you send me your email - I will send you a copy of the document......
Bron