Typically the gerundive is employed when one using a gerund with an object seems possible. For example, I have understood that aqua bibenda est and rei faciendae causa are preferable to aquam bibendum est and rem faciendi causa. It seems that one can always transform a gerund with an object into a gerundive (as an attribute to the object), but my grammar tells that this is not strictly necessary in all situations.
Tuomo Pekkanen's Ars grammatica states that the gerund can take an object only if the gerund is in genitive (without causa or gratia) or in ablative (without prepositions) and in all cases the gerundive can also be employed. For example, one can say spes urbem capiendi or spes urbis capiendae and librum legendo or libro legendo. Instead, one cannot say aquam bibendum est and rem faciendi causa. This rule sounds weird to me, and I wonder how confident we really are that classical Latin uses gerundives with objects in these and only these situations.
How strong is the rule? That is, do we have strong evidence that the Romans always obeyed this rule? Do ancient authors always follow this rule? Do other modern grammarians agree about these limitations to a gerund having an object?