The Latvian, seeded fifth and ranked 24th in the world, was replaced by Muster's fellow Austrian Andreas Haider-Maurer, a lucky loser ranked 157th.
The two Austrians will meet on Tuesday.
Organisers said Gulbis withdrew for personal reasons.
Muster, French Open champion in 1995, was given a wildcard for the tournament.
Ranked 974th, former iron-man Muster began his comeback earlier this year in the lower-level Challenger Tour and has managed one win in seven outings, against Borut Puc in Slovenia last month.
The ATP said the Austrian would become the first 40-plus to compete in the main draw of an ATP tournament since Jimmy Connors played in Atlanta in 1996, also at 43.
"I'm looking forward to it enormously," Muster told local media. "I want to inspire the crowd with my fitness and fighting spirit.
"I'm fighting like in the good old times and I will give everything in front of the fans in Vienna."