Most will interchange. It would be best to get one for your brand. Gas logs like the type that go into wood fireplaces or masonry fireplaces normally use just thermopile which normally interchange also. Some of them also use a thicker thermocouple than a normal pre fab box would use. Everything I have run across so far still interchanges though. Most major brands use a select few valve brands and that determines the thermocouple. Also ventless fireplaces have the thermocouple built into the pilot assembly and if it goes out you have to buy the whole pilot assembly.
One should always use the factory part when dealing with gas fireplaces. Just because a thermocouple fits, that doesn't mean it is the correct one. Also, there are many different thermocouples that have different fittings and different drop-out rates. If someone puts a "universal" thermocouple in a direct-vent gas fireplace, there is a chance that the glass can explode!!!!
Just remember that the thermocouple is part of the pilot safety shut off system, if you don't have the proper thermocouple, then the safety system doesn't operate as designed.
My point by saying to try and get the same brand. Although in my experience e.g. a robertshaw valve uses the same thermocouple regardless of what brand of fireplace it is in. That was my only point. Don't want anyone to think I am trying to get someone blown up. Just an FYI. We had a homeowner in Knoxville with fireplace problems (reportedly) that got up one night because he smelled gas and the next thing he knew his million dollar house was a pile of rubble. It brought the whole house down. Please be careful with gas.