Firstly, Gaitskell was anti EU....as in federalism. He recognised the danger of the "closer union" clause in the Treaty of Rome. So no further attempt to join the EEC...possibly some sort of greater EFTA.
Secondly, he was not a unilateral disarmer. If he became PM in 1964, the Polaris deal would already have been done by Macmillan. ..so that stays.
During Suez he wanted the dispute referred to the UN....possibly his view would be the same on Vietnam. Hopefully he would not have sent British troops.
Also he was the Chancellor who introduced prescription charges....so well aware of the costs involved in Government spending. Military spending would have been subject to a cost/benefit analysis so it is likely that many projects would still have been cancelled.