Firstly, there is no rule of grammar stating that adjectives should only ever be followed by infinitives. For instance, we say 'be busy doing smth.', not 'to do...'.
However, in cases of a postposed verbal subject, to which you are apparently referring, standard practice is to use an infinitive to refer to a hypothetical event, and a gerund to refer to an actual/habitual event.
It would be wonderful to win a million pounds in the lottery.
(because the event in question is simply imaginary)
It was wonderful seeing you again yesterday.
(because I actually did see you.)
That said, however, these are tendencies, not firm rules, and level of formality also exerts some influence over choice of form, with the infinitive being generally more formal, and the gerund generally less so.