Only receiving 28Mbps and paying for gigablast. other neighbors report the same thing.

This issue is widespread.  Cox is not delivering what it promises. I think it might be worth looking into a class action lawsuit? 
All we need is enough people to gather together, which I do not think will be an issue.  It may not be hard to prove that they are selling something that is truly not supported.  
I am sure just by looking at the forums that not everyone's inside wiring is the issue. There are many technical and IT people on this forum.  I do think Cox is selling something it cannot possibly deliver on.  Then on top of it there support representatives treat you like dirt. 

Do you know for sure that your area has fiber?  Have you ever received the approximate speeds you expected?  Fiber or no fiber, your speeds are slow by current standards.  You've received replies from other forum users offering to help you with that.  My questions are concerned with:

1. Did Cox sell Gigablast to a customer in an area that can't support it?
2. Did you upgrade to Gigablast in the hope it would improve slow speed?

  If you find your way back to this forum, please post a reply.

Are you testing over Ethernet? testing with a device connected via Ethernet will be the ONLY accurate method of seeing what speeds are are seeing. There are way too many things that can affect WiFi, for a test over WiFi to be effective. 

What modem/router are you using?

You should receive a Cox email when a service appointment is scheduled.  If you don't receive that email, you probably don't have a scheduled appointment.

Have you ever received the approximate speed you expect?  If this isn't just a short-term issue, your area may not support that speed, regardless of what a Cox sales rep may have told you.  If your area can't provide the speed you're paying for, why don't you change your plan to a speed it can provide?  If the speed you're currently getting is within the limit of the new plan, you should get the same performance for a lower price.‡  When/if your area is upgraded, change back to a plan with a higher speed then.

On the other hand, you may have a signal issue.  If a service tech determines the issue is on Cox's side of the demarc, you won't be charged for the tech visit and the tech will escalate your issue to have the signal corrected.  This will require you to contact Cox again and schedule a tech visit to have your signal checked.

‡ This assumes that on a congested node, (a) Cox attempts to provide the full, approximate speeds associated with each customer's plan.  A second possibility is (b) Cox prorates speeds based on each customer's plan.  Upgrading would provide faster speeds, but nobody on the node would get their plan's full, approximate speeds, not even customers on the lowest plan.

I suspect a lot of people with slow Internet assume Cox uses option (b) and upgrade their plan to compensate for the slow speeds.  If Cox does use option (b), then customers have to decide for themselves if the prorated increase in speeds is sufficient to justify the full increase in price.  But if Cox uses option (a), all upgrading their plan would do is increase their Cox bill.

Does anybody have evidence to confirm which method Cox uses? 

This is a user forum where experienced folks can occasionally lend a hand. If you'd like useful feedback, please let us know  what modem you are using, how you are checking speed (direct ethernet connectionf from PC to modem?) and what your signal levels and signal to noise ratios are.