By default gcalcli loads 7 days on the agenda view. You can specify how many you want if you give it 2 number args, for example:
GCalCLI start and end args
gcalcli agenda <start> <end>
These numbers represent the number in the month.
To load more days you need to pass the current date as the first arg and the the date you want as the second arg, this is what I use in my scripts
gcalcli agenda `date +%Y-%m-%d` `date +%Y-%m-%d --date="+9 days"`
The current date
date +%Y-%m-%d --date="+9 days"
The current date 9 days in advance