To elaborate on John Doucette's answer that you need to look at the
Poisson distribution and that gc will run $f*p = 10$ times
per hour on average, you might want to look at the
Poisson process
as well. This will answer questions such as "What is the
distribution of the time between successive runs of gc?"
The times between successive runs of $gc$ (inter-run gaps)
are exponentially distributed
with an average of $1/(f*p)$ hours $= 6$ minutes between runs. The
standard deviation of the inter-run gaps is also $6$ minutes. Note that
there is no guarantee that gc will run $10$ times each hour or that
the average length of the inter-run gaps measured during one hour
will be $6$ minutes.
Exponential distribution means that the probability is $e^{-T/6}$ that an
inter-run gap will be longer than $T$ minutes.
In particular, a longer than average inter-run
gap (more than $6$ minutes) has only a $e^{-1} \approx 37\%$ chance
of occurring.
The median inter-run gap is $6 \ln 2 \approx 4.14$ minutes, that
is, over several hours, you should expect half the inter-run gaps
to be shorter (and half longer) than $\approx 4.14$ minutes.
Edit: Material added in response to @jbowman's comments
Several different models can be used in this problem. Some
of those mentioned below might have been rejected by the OP
already, but are included anyway.
The main process runs exactly $f = 1000$ times, no more, no less,
every hour at equally spaced intervals of one millihour.
Every $\frac{1}{p} = \frac{1}{0.01} = 100$th run of the main process calls
gc which thus runs $f*p = 10$ times an hour at $\frac{1}{f*p} = 0.1~\text{hour} = 6$ minute intervals. There is nothing random about all this.
There is a call to gc by exactly one of the processes #$1-$#$100$,
exactly one of the processes #$101-$#$200$, and so on, with each call to gc
being equally likely to be any of the $100$ choices. Again there are $10$ calls
to gc every hour, but now the inter-run gap between two calls to gc is random
and can take on value $i$, $1 \leq i \leq 199$ with probability
$(100 - |i-100|)\times 10^{-4}$. The histogram is triangular with a peak at
$i = 100$. The average inter-run gap is $100$ millihours $= 6$ minutes.
Each of the $1000$ runs of the main process calls gc with probability
$p = 0.01$, independently of all other runs of the process. Now the
number of calls to gc each hour is a binomial random variable with parameters
$(f,p) = (1000,0.01)$ and average value $f*p = 10$. Note that there is no
guarantee that gc will run $10$ times each hour anymore. In fact, gc
might not run at all, or it might run every millihour. The inter-run gaps are geometric random variables with parameter $p = 0.01$ and the average
gap is $1/p = 100$ millihours $= 6$ minutes.
The occurence times of the main process are modeled as the arrival times
in a Poisson process with arrival rate $f = 1000$ per hour. Note that this
does not mean that the main process runs $1000$ times each hour: the number
of runs is a Poisson random variable $N$ with parameter $1000$ and average value
$E[N] = 1000$.
Every $100$th run of the main process calls gc. Now the number
of times that gc runs every hour is $\lfloor N/100 \rfloor$ which is
a random variable with average value approximately $10^{-2}E[N] = 10$.
For later reference, I note that the times of the calls to gc do
not constitute the arrival times of a Poisson process with arrival rate
$10$.
Each run of the main process calls gc with probability $p$ independently
of all other runs of the main process. This is called Poisson
splitting and the
times of the calls to gc do constitute the arrival times
of a Poisson process with arrival rate $f*p = 10$ per hour. The
number of times that gc will run in an hour is a Poisson random
variable with parameter $10$ and average value $10$. Once again,
there is no guarantee that gc will run $10$ times an hour even if the
main process runs far more than $1000$ times. The inter-run gaps
for gc are exponential random variables with average value $1/f*p = 0.1$
hour $= 6$ minutes but there is no guarantee that gc will run every
$6$ minutes or so.
Under the Poisson regime, it is possible that
there will be long gaps between runs
of gc, and indeed the main process may well run
once in the first minute in an hour and then $999$
times in the last few microseconds of the hour. However,
this is highly unlikely. If such a phenomenon is indeed
observed several times, a nonhomogeneous Poisson model
(with variable arrival rate) might be considered.
