Attacking units can go all the way up to +150% on attack and defense (+5% per level for each of the three offense bonus GBs, each of which can go up to 10th level).
Defending unit bonuses are a bit trickier. First off, they only gain the bonus on defense, not on their attack stat. Secondly, the defensive GBs give 10% per level, but there are only two o0f them, so that is up to 200%, but in addition you can get a monastery which also adds 20%, and a theoretically unlimited number of watchfires, which give 4% each.
When you start out you won't see too many bonuses to attacking and/or defending units, but by the time you get to the progressive or modern age a fair number of people will have offensive GBs maxed and pretty good defensive bonuses too.