What, if any, non-German NATO forces have been stationed in Eastern Germany (i.e. the former GDR territory) since reunification?
No, the stationing (or deployment) of foreign armed forces and nuclear weapons in the territory of the former German Democratic Republic is explicitly forbidden in the last sentence of Article 5 (3) of the two plus four treaty of 1990-09-12.
This came into effect after the 1994-08-31, which marked the end of the Soviet military presence on German soil. Until then Article 5 (1,2) was in effect that allowed the continued stationing of the Western Allies in Berlin.
Allthough occasionally exercises in (and transit through) the territory of the former German Democratic Republic do take place, there are no (non-German) NATO troops stationed there.
Treaty on the Final Settlement with Respect to Germany (two plus four)
Article 5 (3)
Following the completion of the withdrawal of the Soviet armed forces from the territory of the present German Democratic Republic and of Berlin, units of German armed forces assigned to military alliance structures in the same way as those in the rest of German territory may also be stationed in that part of Germany, but without nuclear weapon carriers. This does not apply to conventional weapon systems which may have other capabilities in addition to conventional ones but which in that part of Germany are equipped for a conventional role and designated only for such. Foreign armed forces and nuclear weapons or their carriers will not be stationed in that part of Germany or deployed there.
Oxford Learner's Dictionaries:
- especially from one of the armed forces, to work in a place for a period of time
- to move soldiers or weapons into a position where they are ready for military action
- Treaty on the Final Settlement with Respect to Germany - Wikipedia
- Vertrag vom 12. September 1990 über die abschließende Regelung in Bezug auf Deutschland (Zwei-plus-Vier-Vertrag) - Auswärtiges Amt
- copy of original treaty, as PDF, in the 4 languages can be downloaded
- German-American Relations - Treaty on the Final Settlement with Respect to Germany (two plus four)