"Gross" isn't a term typically used in publishing contracts, but I'd assume it means getting royalties on the retail or cover price of the book. This is what you can generally expect from commercial publishing houses.
"Net" means royalties paid on the publisher's net receipts--the book's cover price less any discounts, and sometimes various publisher expenses.
Many publishers that pay on net offer a higher royalty percentage, but in most cases this actually works out to no more than, and often less than, a lower royalty percentage paid on the cover price. So net royalties are less desirable, and you need to be especially careful of publishers that manipulate their net by charging expenses against the book's income, which may result in your royalty being paid on a pittance. I just saw a contract from a publisher that pays a royalty of 70% of net--pretty impressive until you read the fine print, which allows the publisher to deduct advertising and promotion, printer's costs, and a menu of other expenses in order to arrive at net. Conceivably, under these circumstances, net could be less than zero.
- Victoria