Similar to Maggie Smith, who had a whole career, but only got famous worldwide when Harry Potter came along, and then even more when she joined Downton Abbey.
I went through his IMDB-list, Mary Reilly is on my list of so-bad-it's-fun movies. Loved Sleepy Hollow. Definitely will remember him for Cranford and Emma. Furthermore, Victoria & Abdul was a nice little movie, as was the King's Speech, small(er) parts for Gambon, but definitely well executed.
I enjoyed him as Dumbledore for some parts, but I think they lost focus in the later movies. With much of Dumbledore's plotline being removed/condensed, there wasn't much to work with. And due to too many memes, I will mainly remember Gambon for this ;-) Now that's a legacy.
And I am now wondering who will play Dumbledore in the HBO series.