MC={'рыба': ['100'], 'мясо': ['120']}
RC={'рыба': ['3'], 'мясо': ['2']}

GC={'мясо': 240, 'рыба': 300}

CG = {}

for key in MC.keys():
    try:
        CG[key] = [str(int(count) * int(RC[key][index])) for index, count in enumerate(MC[key])]
    except KeyError:
        CG[key] = MC[key]

In [8]: MC={'рыба': ['100'], 'мясо': ['120']}
   ...: RC={'рыба': ['3'], 'мясо': ['2']}
   ...:

In [9]: foo = lambda dct_1, dct_2: {key: int(dct_2[key][0]) * int(dct_1[key][0]) for key in dct_2}

In [10]: foo(MC, RC)
Out[10]: {'мясо': 240, 'рыба': 300}

import itertools, operator, functools

MC = {'рыба': ['100'], 'мясо': ['120', '5', '6']}
RC = {'рыба': ['3'], 'мясо': ['2']}
CC = {'рыба': ['2'], 'мясо': ['1', '3'], 'квас': ['3']}

c_dicts = MC, RC, CC

GC = {k: [functools.reduce(operator.mul, map(int, nums), 1)
          for nums in itertools.zip_longest(*[dt.get(k, [1]) for dt in c_dicts], fillvalue=1)]
      for k in set(itertools.chain(*c_dicts))}

# {'квас': [3], 'мясо': [240, 15, 6], 'рыба': [600]}