It really depends on what you have.
Upgraded Gew.98's (Gew.98M) come in many different forms, some are worth something, some aren't worth much. As these are all refinished, by the Germans at least, some do not care for them, while others find them fascinating due to their varied role during the interwar period, various types of reworks, sometimes unit markings.
The real issue is whether it is matching, German matching, or if not how badly mismatched. Finding completely matching ones are not too easy, usually they are bolt m/m rifles, but if the stock is matching and decent that is a plus and makes it have some value. After that it depends on unit markings, what type of rework, early ones are better, - if it has MO (S/42) rear sight parts they are not as appealing to me, earlier dual sights and Simson parts are a plus in my book, but everyone has their own likes and dislikes.
One of the main things to concern yourself with is making sure it isn't a Spanish import, look for a shorter barrel, metal painted black, or really rough condition. Also take care and look to see that some parts are matching, many of the Albanian imports are real rough, bad bores, and usually badly mismatched, though nothing as bad as the rc, which has no collector value imo.