"Hardcore" usually refers to a person or a group who is very committed to their beliefs, or has very strong beliefs. For instance, I might say, "I am a hardcore rock music fan," or, "I am a hardcore hiker."
It implies that the person or group is so committed to their beliefs that it would be almost impossible to change them.
Hardcore should be spelled as one word, or with a hyphen (a dash) between the two words (hard-core).
You can also use the word "diehard", and it means the same thing.
Hardcore can also refer to very explicit pornography or music - in this case "diehard" does _not_ mean the same thing.
I'm not familiar with the phrase "hard gear". Do you mean "high gear"?
Hope this helps!