I must admit I have not used the Gymboss timer in a gym. It's a simple, easy-to-use programmable timer. You set an interval (e.g. 20 minutes), choose an alarm (audible or vibration) and press "start". It will then alert you once the time is up; choose "auto" mode and it will keep doing this every 20 minutes.
I've used it for two purposes:
1. My chiropractor tells me I should be stretching every 20 minutes. The timer is just the job to act as an automatic reminder.
2. When I'm giving a talk or presentation (part of my job), I set the timer to buzz to remind me to change topic / wind up / whatever. It's a great antidote to long-windedness and it allows me to pace the talk exactly the way I want to.
The only negative comment - I would recommend throwing away the "free" battery that comes with it and replacing it with a decent battery.