At 15 months she's probably too young to really grasp "fire hurts" without directly interacting with the fire, and you REALLY don't want that, so you'll need to attenuate the dangers and teach as if the gate weren't there. Put the gate back up and sternly tell her 'NO' when she gets near it. Use the short attention span of the toddler to your advantage; try combining 'NO' with a distraction such as playing with a different toy. Make sure that the 'NO' part is plain and boring, and the distraction is fun and intriguing.
When you are at a home where there's a fireplace and no baby gate, station yourself or another adult near the fireplace where she can be easily intercepted and redirected. Make the interception as boring as possible: no picking up and swinging, no 'happy' redirection like tickles or giggles, just a boring 'NO' and a turn-around to point her in the direction you want her to go.
Sounds like she's getting the reaction she wants: attention. Since she's 15 months old, and timeouts might not really work yet (at least, they didn't kick in for either of my two until they were closer to 30 months), you'll probably need to go back to prevention until she can really grok what you're telling her.
Anecdata: my son (now 32 months) was fascinated by the oven and stove and always wanted to touch it. We handled it by saying "HOT! NO!" when he'd get near it, and mimicking pain (wincing, shaking the 'burnt' hand, etc). He quickly grasped that we didn't want him to get near the stove or oven when it was working, and would back up and say 'hot! hot!'. This was around 20 or so months, IIRC.