Standardgateway is the own IP +1

Hello,

I have the following phenomenon. 
We use FortiClient and when a user logs in he gets e.g. the 23,156,25,153. 
The default gateway is then the 23,156.25.154 (always +1). 
In the FortiAnalyzer I get the message that xx.xxx.xx.153 wants to communicate with xx.xxx.xx.154 on port 8013. 
What's this? 
Can I somehow switch to another standard gateway? If so where I do that?

Thanks in advance for answers

many Greetings
Tobias