Can an AES-GCM be cracked if the key is known but the nonce isn't? I'm kind of new in this field and I've been trying to learn it. 

I'm now facing an issue where I have a couple of packages encrypted with AES-128 GCM and I have the key that was used to encrypt these packages. However, I dont know the nonce or the H. 

Is there any way for these packages to be decrypted? or is the nonce not being known keeping them safe in a case like this? 

Would appreciate all help. Thanks a lot!
You'll have to guess the plaintext of at least one block (128 bits) that is on a 128 bit boundary from the start of the GCM ciphertext. Then you can XOR it to retrieve 128 bits of the key stream.

Now the key stream you can decrypt since you have the key. This will get you a counter value. The counter in GCM mode starts with a 12 byte nonce and the counter as four bytes: 00000002 in hexadecimals. Once you have that nonce you can generate all the other required nonce + counter values.

So yes, it is possible, but only if you can guess at least one block of the plaintext.