Gloss differential is a visible difference in the glossy surface of a print across areas with different amounts of ink or between areas of the page printed on versus not printed on. It's not quite the same as bronzing (see here) but closely related and due to the same general issue of differing amounts of absorption of ink by the paper's coating.

Generally, it's not a huge problem as it's not easily visible, but it can be visible in direct viewing of the print and can create an unpleasant appearance.

In general, spraying your prints with a high quality coating will substantially reduce or even eliminate gloss differential with typical semi-gloss and gloss papers.

With super high gloss papers (i.e plastic based papers like Pictorico High Gloss White Film), the spray does work but will leave a visible semi-gloss texture on the print and it isn't an ideal solution. The best option is to try another paper but the problem is inherent in current coating on high gloss papers and there is no perfect solution at this stage.