Water backing up in your guttering can sometimes be as much about blockages in the downpipes as the gutters themselves.
If water cannot drain quickly through downpipes to ground level, there's nowhere for the rain to go, so it begins to gather in the gutters until the rain subsides, falling more slowly than the waste rainwater drains, or it begins to overflow to surfaces below.
Professional gutter cleaners will be able to assess blockages and advise you whether it's possible to clear a blockage using specialist equipment or in rare cases the downpipe may need to be replaced. Generally, replacements are only required if blockages have become compacted and are unable to be shifted.