I understand that by ww1 lmg's were already made and was far superior than the 18th century gatling but as stated above, if an 1800's gatling gun was to be mounted on a ww1 fighter for use in both dogfights and air support how would it affect the plane?
Using a Gatling gun on a WWI aircraft would be subject to the same problems as using any machine gun on an aircraft:
Preventing the propeller from being shot off by the gun. Recoil-operated MGs used interrupters that delayed firing when the propeller was in the firing line, while a Gatling could be driven off the driveshaft, and be geared so it never fired at the propeller. Some new way of initiating feeding and firing ammunition would be required, since gatlings cannot not fire a round and not eject it afterwards anyway.
Weight. WWI aircraft were light, flimsy and low-powered, and payload was at a premium. Gatlings are big, bulky weapons that were mounted on wheeled carriages, while the MGs that were mounted on aircraft could all be fired man-portable (if only barely in some cases). Basically, they're too heavy. A special plane would have to be designed around it. It would require a bigger engine (when metal was expensive) using more fuel (also expensive).
Rate of fire: Early Gatling guns did not have a rate of fire significantly different to single barreled MGs. The multiple barrels were intended to reduce the thermal load on barrels that were of lower quality metal than modern barrels. This could also be achieved in less weight and complexity by water cooling. The canvas ammunition belts of the day did not lend themselves to higher rates of fire without significant risk of jamming.
So, a Gatling could only be mounted on a bigger, less agile, less efficient and more expensive aircraft than other WWI fighters, and would only be so placed where no other alternative weapon existed.
The resultant fighter aircraft would be... suboptimal. Even for WWI.