Here is kinda how I se it, a GMRS radio can not be modified to operate (transmit and receive) on Ham bands, But a Ham radio can be modified to operate on GMRS bands. Some CBs can be modified to operate in Ham Bands, (free bands) and that is highly illegal but Ham operators often use the CB frequencies, and it's OK.
I will admit I did NOT google all of the radios mentioned in this thread, I don't always understand the acronyms used in the forum, but the more I read, the more I learn.