gpicview is a really simple image viewer which lets you easily browse the images in one folder and that's it. Changing folders isn't easy; you have to choose "open image" and navigate the pile picker and pick an image in a folder. That alone makes it painful to use as a general-purpose image-browser. There is no folder tree view or other easy way to switch folders. gpicview hasn't been updated since version 0.2.5 was released in 2016.
Features and usability
You can fit images to window as long as they are larger than the window. The zoom-to-window setting is remembered.
Navigating the images in a single folder is easy, you can use space and the arrow keys (up or left and right or down for next/previous). You have to use the mouse and open the file picker and choose an image in another folder to switch folders. That's not a very quick or easy way to switch between folders. If you're fine with that then perhaps gpicview is for you.
You can do the most basic image editing: images can be rotated and flipped vertically and horizontally. And that's it.
gpicview is fine for browsing the images of just one folder. It's nothing for use-cases beyond that and it's not really very usable as a general-purpose image-viewer.
gpicview was developed as part of LXDE which is now abandoned for LxQt. Thus; it appears that further development of gpicview is unlikely.