This is an interesting gift.

All gifts have their ulterior motives. In this case, the motive is upfront:

http://timesofindia.indiatimes.com/world/pakistan/china-gives-pakistan-two-ships-for-security-of-cpec-sea-route/articleshow/56558924.cms

It is not only the China needs the security of such sea lanes, but as a co-trading partner, Pakistan also wants to be able secure its sea lanes.

So this is a win-win for both parties.