When in doubt I ask “what’s the worst that could happen”…when the answer is “the house could burn down and worse”, it’s better to be safe than sorry. I’d call in a professional chimney/inspector…hopefully it can be a quick turn around.

That hardened stuff in the flue is creosote. That can cause chimney fires. I would not use it until it was cleaned and inspected.

Gas fires do not produce creosote, but they do produce other buildups. Have it checked and cleaned to be on the safe side. You never know, it could of been converted from a wood fire.

I would have the chiminey cleaned and inspected, and I would have the gas logs and the connections etc all inspected. It hasnt been used for a long time, and I would wonder if there was a very good reason it hasnt all been used.

OP said gas logs were installed by the owner before the previous owner so I assumed the fireplace was once wood burning so that is why I mentioned creosote. No gas fireplaces do not produce creosote but what is in that flue looks like creosote.

Have a chimney inspector or a experienced chimney sweep take a look at it make sure it's clean and properly vented for gas. (I thought a smaller diameter liner had to be installed???) Maybe the black is old stains from previous fires and not a creosote build up. Much of the chimney still showing the orange of the terracotta lining and not stained so must not have been too many wood fires.