When you import your data, using OFX, or CSV file format, you will have to re-categorise your transactions from the default Credit, and Debit categories.

Once you have categorised your entries, any future imports you perform for the same Payee, will be automatically categorised.

If you make and receive payments to the same payees, it would better to import a small selection of entries, then categorise these first, before running a large import