GIMS is just an IC overlay.  Once you create the IC items, you can add them anywhere.

We created a small suite of customized screens / forms for grid-based item creation, adding to a P/O and adding to a Sales Order.  The format in use is SKU-COLOUR-SIZE.
As Jay indicated, GIMS makes use of item segments to manage the colour and size dimensions.  

I already have an add-on that allows me to create items, PO's O/E and reporting in grid format. 

I am asking specifically about using a grid format to add items to the material category in PJC. My current GIMS does not have that option, wondering if anyone has a solution that addresses that limitation. 

Thank you

I highly doubt such a solution exists, there's no market for it.  If you have programming skills, Orchid Extender could do it.