I have been working with GPS systems for 4.5 years, most are either written in either Java, C++ or C# this solution is Java based, CentOS 7, Red Hat are the recommended operating systems.
What I suggest is to use GTS Enterprise (GTSE) which is a commercial GPS system.
Basic GTSE is distributed with the source code so it can be modified, and optional modules for example Device Communication Servers (DCS) or Enterprise Notification Rules Engine (ENRE) are distributed as compiled .jar files, and the source is not available.
As far as ownership of the source code is concerned, the copy right holder of the source code belongs to the developer, you can not resell, or distribute it.
If your intention is to support multiple companies, with multiple domains, logos, styles, etc... GTSE does support that, a single installation of GTSE can support multiple companies. The limitations are the amount of drive space, and memory the host has, the more device the more resources are required.
Since there is limited space for the proposal, only 1500 characters, I preparing a copy of your requirements, with my thoughts, and comments. The document can not attached to this proposal, but can be uploaded through the message board.
Please feel free to contact me on the freelancer message board to discuss your requirements further.