GPRS-devices can connect to server only by ip.
They are permanently connected to the server.
How to prevent Denial of Service, when internet-provider's channel is down?
Is there any standart solutions for this situation?
It's very difficult to understand your question but I'm going to assume you mean that you have a number of devices that need to be permanently connected to your servers and you want to be able to maintain service in the event of your GPRS link provider having an outage - is that right?
If so then you only have one other option, have a backup GPRS link provider and enable your clients to detect primary link failure and automatically switch to the secondary link provider (and later back to the primary).
There's no shortcutting this, it's actually a valid question and answer regardless of the transmission protocol - the same is true of ethernet, fibre-channel, wifi etc. If you need to survive the failure of any form on network you have to implement a secondary path using physically and logically different components and service providers where appropriate.