For me, it's is a hassle... but the tool I use is Google Earth.
I've customized a set of data I originally obtained from what is NOW maintained at
https://tnlandforms.us/landforms/
I then create a new Google Earth path, clicking along the trail system from the above mentioned data... because Google Earth sucks when it comes to editing the data (i.e. no simple way to combine a set of existing paths).
Then I can use Google Earth's 'Show Elevation Profile' to see what the trail is going to be like.
I keep telling myself I'm going to one day write a program that would allow me to select a series of GSMNP trails and the program would generate a custom Google Earth Path from the known paths... but well you know how that goes.