It depends on how much the gain is. The executors have an annual allowance of £10,600 (I think - check this on HMRCS website). You can "roll up" the allowance for the year of death and following two years if the property does not sell in that time.
However, assuming it sells next week for £50k in excess of probate (unlikely I know), yes there will be CGT payable.
There is, however, a way to mitigate this. IF and only IF you are able to settle all her debts from assets other than the house and residue is therefore "ascertained", you can transfer the equitable interest in the property equally to all the beneficiaries and sell it on their behalf. That way, the gain is in the hands of the beneficiaries and NOT the personal representatives. If the beneficiaries each have unused personal CGT allowances there may be none payable. It can be quite tricky to get this right so it might be worth taking legal advice.