Gray is more desired by other teams because he is not eligible for free agency until 2020. Darvish is a free agent after this season and thus qualifies as a "rental."
The Rangers have made the decision to find the best deal possible for Darvish, but aren't committed to trading him if they don't get the desired return. The Rangers still have interest in re-signing Darvish after the season.
Another thing to remember regarding the Darvish market are the new Qualifying Offer rules that were set forth by the Collective Bargaining Agreement that was ratified during the offseason. Under the new QO rules, only teams that receive revenue sharing can receive a first-round compensation pick when a free agent rejects a QO to sign elsewhere.
According to MLB.com's Jon Paul Morosi, the Rangers are not on the list of 16 clubs that will receive revenue sharing, which isn't surprising given the size of their market. As a result, the best they can do if Darvish rejects a qualifying offer and leaves is a comp pick before the third round (likely in the 70s). These new rules could incentivize a trade, as Darvish would certainly bring back a prospect package more valuable than a Draft pick in that range.