Why does the lady say her guest cannot possibly be Sir Gawain?

The Lady says that if he were truly Gawain.... he would have craved a kiss.

But ‘God bless you! For this reason’, blithely she answered,
‘that one so good as Gawain the gracious is held,
who all the compass of courtesy includes in his person,
so long with a lady could hardly have lingered
without craving a kiss, as a courteous knight,
by some tactful turn that their talk led to.