If I want a token, say GNT, that is not listed on exchanges and I missed the presale, what are my options?
You can get GNTs from https://cryptoderivatives.market, a decentralised trustless exchange contract market. Send a small amount first to a contract address to test and check you receive the correct amount of tokens in your sending address. And don't send from an exchange wallet, but from a wallet you control, e.g., using Ethereum Wallet, https://www.myetherwallet.com or Parity. Users don't pay any fees except for the standard Ethereum transaction gas fees.
This site will soon allow for the listing of purchase and/or sale contracts on ERC20 compliant tokens.
Disclosure: I run this website.
Also, there is a 108 ETH Bug Bounty on the new upcoming smart contracts.
See Golem Network Token on coinmarketcap.com (CMC). CMC lists the markets GNT is trading on and the volume. Currently, Liqui has the highest volume and might be your best bet. GNT also trades on mkr.market, which is a decentralized exchange (not tracked by CMC). It should also be noted that Julian Zawistowski of the Golem Project just submitted a request to Poloniex to have GNT listed, so it should trade there soon.
Consider using CMC to research any other token. If it's not found on CMC, try to discover the token project's subreddit - it will typically list any exchanges on the side bar. If there are no apparent exchanges, the community may simply trade the token in the sub by posting seller/buyer threads and using trusted community members for escrow.