I do not know if that is true (I don't sell GTL), but I do know that you better check with your E&O carrier if it is. My policy, at least, requires me to use only A- or better carriers.
This is not the first time I have heard this kind of thing. I have scoured my E&O policy and see absolutely no reference to the required level of rating for the companies I am contracted with. I don't know who your E&O company is.
I called my E&O rep and had them confirm my reading of the policy regulations. They reported to me that the only time the policy did not cover me, is if the insurance company's policy I sell is in BK. This is the way I read it, too. This is reasonable.... I would in no way sell a policy to my customer from a company I knew to be financially unsound even if they were not in BK. (e.g. This is why I will not sell PTNA LTCi, although I would have E&O protection if I did. PTNA has been flirting with financial problems for the last several years but has not filed BK... yet! In my mind, LTCi PRESUMES long term viability.)
Perhaps when renewing your E&O, you should consider BCS of Oakbrook Terrace, Illinois. In today's economy, a company rated A- today could be B- tomorrow. If you have trouble sleeping at night because of this, here is a solution.
( For What It's Worth: BCS is rated A- by AM Best... at least today it is!)