Hello,
Guest Management Software is still available. Going to the download section of the MSM775 you can find it in the section "Other" and you can download it if the controller is entitled to software support.
https://h10145.www1.hpe.com/downloads/SoftwareReleases.aspx?ProductNumber=J9840A
The latest release is Guest Management Software (GMS) V2.0.2.0 from 18-Apr-2016. Starting with Version V2.x there is no requirement that the versions of GMS and the MSM controller should match.
If you have experience with configuring guest networks with other MSM controllers especially with MSM765 there should be no difference with MSM775. If you are using multiple controllers in team, you have to be aware of some limitations, for example you cannot use the controller as a DHCP server and you can use only a single option for DHCP Relay called "Extend VSC egress subnet to VSC ingress subnet".
http://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-c04514114-1.pdf
You can consult the configuration guide, especially
chapter 20 Public/Guest network access
Chapter 14 Controller teaming, subchapter Guest Access and Teaming
Here is the GMS V2 Administrator Guide