Based on Craftpix.net's license for the assets you've derived these from, this shouldn't be relicensed GPL. Craftpix has released the source assets under two licenses: 1.) A proprietary license specific to Craftpix.net as seen here: https://craftpix.net/file-licenses/ 2.) OGA-BY, a DRM-waivered version of CC-BY, as seen here: https://opengameart.org/content/level-map-2d-backgrounds There is no problem sharing these derivatives, since either of Craftpix's chosen licenses fully allows that, but you should license these derivatives with the same license you've acquired them under. GPL is niether the Craftpix Custom License nor OGA-BY and may have some incompatibility problems. I don't know how useful the above derivatives are as game assets, but hosting them here, I would advise changing the license to OGA-BY since the Craftpix Custom License is not an available option here. I would also advise linking to the OGA-BY version (here on OGA) instead of the craftpix.net version to avoid licensing confusion for users.

I'm not sure that's true; if I understand correctly, OGA-BY (like CC-BY) does not require derivatives to be distributed under the same license (i.e. it allows sublicensing), as long as attribution is maintained. Since GPL requires attribution as well, and CC-BY is GPL-compartible (see https://www.gnu.org/licenses/license-list.en.html#ccby ), licensing derivatives under GPL should be fine. OGA-BY could also be added as an option, but it's not required. 

If the original were only under CC-BY-SA 3.0, this would not be allowed, since CC-BY-SA 3.0 is not compatible with the GPL. Interestingly, CC-BY-SA 4.0 is "one-way compatible" with the GPL, meaning CC-BY-SA 4.0 works can be sublicensed to the GPL v3: https://creativecommons.org/2015/10/08/cc-by-sa-4-0-now-one-way-compatib... . 

Ah. I think you're right, bluecarrot. Although GPL doesn't *require* attribution. It does require the source to be indicated, and it allows the author to *request* attribution. Regardless, the author has requested attribution so yes, my mistake.

Since this is not the original, but the original is being attributed (and the source indicated), this is all good. Disregard my concern above. :)

@InnerClonflict: You don't have to do anything. You can change it back to GPL if you prefer, though I believe OGA-BY is a far more useful license for game art. It's totally up to you, though. Sorry for the confusion.

As for the art/illustrations/photos that you've tweaked, it depends on what the license is for those art/illustrations/photos. The license may allow your derivatives, and we'd love to see them. :) If you can't find a license for the art/illustrations/photos you've based your derivatives on, it probably means the license is proprietary and your derivatives are considered "fan art" which, unfortunately could not be hosted here. Still, it's working checking into. :)

@InnerConflict: you can do whatever you want, depending on how you want people to handle your work; for instance, GPL would require anyone who makes derivatives of your work to also license their derivative under GPL; same with CC-BY-SA. CC-BY just requires you and the original author be attributed; same with OGA-BY, which also specifically allows the work to be used in a game with DRM. See more info here: https://opengameart.org/content/faq#q-proprietary .  I generally try to make my work available under as many licenses as possible, to give people options, and I encourage others to do the same!

Cool. I'm not sure I understand the distinction between "requiring attribution" and the requirement "that the source be indicated." Everything I've read suggestes the GPL requires attribution: https://opensource.stackexchange.com/questions/4577/does-gplv3-require-a...

But at this point we're being pedantic :) 

attribution = *author + *license + (optional link) + (optional limited message)
source indication = *license + *link + (optional author)

but yes, I'm being pedantic. :P