Here's a quick-and-dirty comparison example. I used Photoshop; GIMP should offer similar options.

Here's our model, a P90 I randomly snagged from FPS Banana. Oh, but it's too large for Doom, in fact it'd take up almost the entire screen, so let's size it down...

The easiest resize would be using a Nearest Neighbor filter, which makes it so you don't have to trim down the edges to avoid having weird blue glowy stuff around the gun in-game. But...

...if we zoom in on it, to about what size it'd appear in-game, you'll notice some areas (I've circled them in red pencil) that end up jagged and ugly-looking. This is the telltale sign of a model rip.

In this picture I've opted to size it down using the Bicubic filter. GIMP should have this in the image resize options; you might also see a "Lanczos" option which produces nicer results as well. I've colored the background differently, and I'll explain why in a second. Look at the three areas indicated by the last picture - the etched lines on the magazine now appear smoother and more natural, the sights are better defined and look less like a blotchy blue shape, and the glove looks less like it could amputate the hand. But the main pitfall (and reason why I colored the background another color) is that you'll have to trim the edges a bit by hand to make sure the weapon doesn't appear to have a blue glow in-game.
Hope that illustrated it well enough.