From the description of the -m option:
gsutil performs the specified operation using a combination of multi-threading and multi-processing, using a number of threads and processors determined by the parallel_thread_count and parallel_process_count values set in the boto configuration file. You might want to experiment with these value, as the best value can vary based on a number of factors, including network speed, number of CPUs, and available memory.
If you take a look at your .boto file, you should see this generated comment:
# 'parallel_process_count' and 'parallel_thread_count' specify the number
# of OS processes and Python threads, respectively, to use when executing
# operations in parallel. The default settings should work well as configured,
# however, to enhance performance for transfers involving large numbers of
# files, you may experiment with hand tuning these values to optimize
# performance for your particular system configuration.
# MacOS and Windows users should see
# https://github.com/GoogleCloudPlatform/gsutil/issues/77 before attempting
# to experiment with these values.
#parallel_process_count = 12
#parallel_thread_count = 10
I'm guessing that you're on Windows or Mac, because the default values for non-Linux machines is 24 threads and 1 process. This would result in copying 24 of your files first, then the last 1 file afterward. Try experimenting with increasing these values to transfer all 25 files at once.