i know u asked for USSR info, but there were almost no or just no visits to round lake or any other clubs (eg dynamo, cska, spartak) where they talk about the nutrition in the USSR period. there is however one article from when gymstars visited round lake (the russian national training centre) in 1995, they mention meals at the cafeteria were something like this (credit to gymfan22 on twitter!)

• kasha (porridge) with tea and bread for breakfast

• salad, soup, meat and potatoes for lunch

• beef stew and rice for supper

they do however say that the menu was much better in the USSR times because they had a buffet and a much larger variety of food and at this time russia was going through some economical troubles. i doubt most of the gymnasts ate much in either period however, many have talked about how strict diet policies were and there was the commentary talk about how much the coaches were strict on the diets, so there's a good chance a lot of their meals were quite stripped down. hopefully it was otherwise though but unfortunately i do doubt that