TGC doesn't require you to mention AppGameKit in your game (although it would be a nice bit of publicity for them if you did). You can use the ogg music format without attribution.
As for other assets such as graphics, music and sounds, terms and conditions for use vary very widely. The website where you bought your assets will have a licensing section which states how you can use them, and whether you need to credit/link to the website. Most paid assets bought 'royalty-free' don't need attribution, but it's worth checking - and emailing to ask the owner if the terms are not crystal clear.
Be aware of 'free' content, as there are sites which will offer 'free clipart' and such as clickbait. The owner of the assets in these cases is sometimes unclear, and could cause you problems later on. (I'm not saying this is always the case, but it pays to be smart about it.)
Hope this helps,
James