Εγκυμονώ, as you know, is a verb, not an adjective. Apart from the standard meaning (Εγκυμονεί = She is pregnant), it is used figuratively: Η κατάσταση εγκυμονεί κινδύνους. The situation is fraught with danger.
As regards the two adjectives, you must consider the very basic difference in register. Έγκυος is the standard word, γκαστρωμένη is colloquial or slang, and also rare, with the exception of the expression "Όλα του γάμου δύσκολα κι η νύφη γκαστρωμένη" (i.e. There's nothing easy in this affair).