“Without the transparency effect” could mean a dozen things. You can hide it, you can exclude it from editing, you can decompose the image into separate layers, you can move it to a layer mask, you can underlay a solid color, and you can totally remove it (RGBA to RGB). I don’t know Photoshop, and I don’t know what feature(s) you’re trying to reproduce, but I’m sure you can do it in GIMP.
The picture you uploaded in this post looks like you added a layer of white below the original image and merged them together. To do the same, I already said “…just add a new layer of pure white below all the other layers…” If you want to destructively remove the transparency, merge those layers. Alternatively, you can select your desired background color in the toolbox, right click the layer, and select Remove Alpha Channel. That will destructively remove the alpha channel by underlaying the background color you chose.
I’ve tried to explain the various aforementioned features. Do not speed-read my posts. Watch Radar’s video. If none of this is quite what you’re looking for, you’ll have to explain in more detail what you want to do (or not do) with the alpha channel.