I texted from my email address when the person replies it goes to my iPhone ?!?

I often use my email (just a regular old gmail account) to send SMS/text messages to coworkers. I do this so I don't get bothered on my personal phone with their work-related replies.

However, all the sudden (seems since OS 6 maybe?), when my coworkers (also using iPhones) hit reply, it comes through as an iMessage on my iPhone instead of going to my email. I tested it myself, and sure enough, when I tried to reply to an email from me - it converted to an iMessage without choice and sent the message to my phone instead of repling where the message came from!

Is this a setting on the new OS that I can somehow turn off? I'm I the only one who wants my replies to go to the place they were sent from?

I removed the work email from the iMessage settings and now it says "This person cannot be reached by iMessage at this time"

I disabled iMessage on my phone and now when I try to reply it says "iMessage must be enabled to send this message"

Help!

(Thanks in advance)