For all we know the key is "dog".
You didn't say what the key is, or what the encryption algorithm is; the only information given is the cleartext, "hello", and the ciphertext, "GH6SDgsd2".
If the cleartext is changed, the resulting ciphertext will change.
If the key is changed, the resulting ciphertext will change and, again, there's no telling what the resulting ciphertext will be without knowing 1) the key, and 2) the algorithm.
Encryption software works by first converting the cleartext to a series of numbers (in a computer, text is always stored as a series of numbers). Then, the software performs one or more mathematical operations on these numbers, the operations performed depend on the encryption algorithm used by the encryption software. Common encryption algorithms include AES, Blowfish, etc. The key is a separate series of characters, or numbers, that are also used in these mathematical operations. If the same cleartext, the same key, and the same algorithm and encryption software are used, the resulting ciphertext will be the same.
Ciphertext is sent as a secret message to someone. If the receiver of the message knows the ciphertext, the key, and the encryption algorithm and software, the receiver will be able to decode the message.
If someone in possession of the ciphertext doesn't know the key, or what encryption algorithm is being used, they won't be able to decode the message, or at best it will be very difficult for them to decode the message. The difficulty involved in attempting to decode a given piece of ciphertext, without having the key, depends primarily on the length of the key. If the key is long enough, it might take many years for someone to decode the message.