GRE was a proof of stake coin originally intended to bring awareness to the carbon footprint of the world and cryptocurrencies themselves. In this coin's ecosystem, renewable energy producers were incentivized by being rewarded with a Greencoin investment for their contributions. These coins could also be staked using the POS model to earn additional income in the form of stakes by further supporting the network.
The GRE project has since been abandoned. While there is some activity in the project's forum page hoping for a reboot of this particular blockchain project, the outlook is not good. There are still some people staking their coins and a few places where they can still be traded, but the volume is quite low everywhere.
If you were trying to sell your investment in this cryptocurrency you'd likely have a very, very long wait ahead of you. The current website holds no real information, but if you'd like to relive the history of this cryptocurrency, then you can try the internet archives to view it in its former glory.
If you're looking to buy Greencoin, then good luck. This asset is only listed on a couple of trading platforms, and the volume is non-existant. If you really, really want it for whatever reason, then your best bet is likely to try to execute a peer to peer trade on a forum such as bitcointalk or some social media platform where you may find a holder that has some they'd like to be rid of.
The only place to store your coins would be in the official wallet download. However, be aware that there is not much activity surrounding this asset, so many links for wallets may be dead ends. You also may have great difficulty getting any support should you need help with anything regarding the wallet. Consider all software as is, and with no hope of future upgrades.