Using a Binomial distribution with number 7.5 billion and probability 0.50, the chances of Earth's death count from a universal unbiased killing resulting outside of 49.99%-50.01% of the total population is 1.7*10^-132, which is less likely than flipping 438 heads in a row with a fair coin.
Updated: I was first using a Poisson distribution which is a limiting case of a Binomial distribution (old claim: The chance of >50.01% (or <49.99%) of Earth's population being killed universally randomly is 3*10^-32! This is less likely than flipping 100 heads in a row with a fair coin). I found this site now which can handle the very small numbers involved in the use of the error function.
So, it doesn't really matter which interpretation is the true case: Planets with realistic populations and even very accurate census won't notice the difference.
Full disclosure: I haven't seen the movie, nor have I ever read a comic book or know who any of these characters are. But I'm familiar with statistics and probabilities and have a habit of browsing the network Hot Questions, so...
Explanation: The Binomial distribution is useful to see the results here. It gives the probability of observing integer random outcomes (like number of people killed) from any expected average (like half of a population). My use depends on the following assumptions.
* The gauntlet will kill exactly half of the universe's (sentient?) lives (rounded if total is odd).
* The gauntlet will take absolutely nothing into account -- the chosen to die are chosen randomly to meet this quota. This means the gantlet does not travel planet-to-planet, or choose those to die one at a time, or kill the oldest, the smallest, the smelliest, etc.
* The population of any planet is a small fraction of the universe's population. If Earth comprised all of the universe, obviously exactly half of us would die. 921
But if 10 googolplex exist, and thus 5 googolplex must die, each of our deaths makes a negligible impact on the quota and our chances of living are unchanged -- 50%.
* The Marvel universe has no more than a trillion trillion trillion times as many galaxies as our own universe, and each of its stars has no more than a trillion populated planets each. These very large (and very small) numbers can be hard to wrap your head around, but no scientific estimate of the number of galaxies/stars/planets in this universe comes close to legitimizing the chances of the asymmetric planet-wise killings proposed in the comments/answers.
The Math: From the binomial wiki page linked, the standard deviation is $\sqrt{np(1-p)}$, with p=0.50. For the Earth (n=7.5 billion), this is 43,301 people. In terms of sigma, 0.01% of our population (7.5 billion) is 17.32 sigma. As a rule of thumb for statisticians, there is a 99.7% chance of drawing a number from within three sigma from the mean. Likewise, there is a 99.999999...% chance of drawing from within 17.32 sigma from the mean. Put another way, with the error function tool linked above, the chance of drawing outside of 17.32 sigma from the mean is less than 1 in 10^100!
The range of likely death counts from this event is much smaller than Earth's census would even notice. Moreover, as others have explained, the "kill randomly universally" method also avoids the complication of identifying to which planet (or ship, or rock, or floating naked in space...) each person belongs. 921