The Discovery Hotel, Mercure and Roma I and II all allow guests with ID. Mercure may charge a fee for guests (not sure, the policy changes every now and then) but last I checked it was $20 USD. One way to try to sidestep that charge is to book the room for 2 but that has had mixed results. The good thing about the Mercure is that hotel guests get the great free breakfast in the downstairs patio while everyone else pays, so it somewhat evens out.
The Romas and Discovery are well-known in the straight sex tourism community as hotels that are friendly to male tourists there to pick up women discreetly, and gays that are discreet can easily use the place and check in their Dominican guests for a visit as long as they have their cedela (ID) which is left at the front desk and/or copied there. These three places are no frills and very simple but the Roma 1 and 2 do have rooms with kitchens available and every room has a TV and small refrigerator.
I would describe all of these places as "friendly to single travelers" instead of "gay friendly", because some more culturally naive posters might get the wrong idea about the establishments or about the DR in general.
All of these places are along the Conde, either the exception of Discovery which is one short corner off the Conde but adjacent from Grand's / Paco's, the 24-hour restaurant / pick-up spot.