Something isn't right!
You say that the Gertrude working in Hackney in 1901 is your great aunt, yet you give the same birth year of 1883 for your aunt Gertrude as well. Are you confusing two separate Gertrudes?
With minimal detail given it's a bit difficult to be certain. but the 1891 census has the following family
RG12/1284 folio 121B
99 Pineworth St, St Andrew the Less, Cambridge
Henry Marshall head marr 40 Commission agent, coal & corn trade b Cambridge
Eliza Marshall wife 32 dressmaker b Thetford Norfolk
Alice Cecil Marshall dau 12 b Cambridge
Olive Marshall dau 8 b Cambridge
Gertrude Marshall dau 7 b Cambridge
Henry Marshall son 5 b Cambridge
Ada Marshall dau 4 b Cambridge
Infant unnamed dau 3 weeks b Cambridge
In 1901 this family is at 9 Hobart Rd Cambridge
Henry Marshall head marr 54 firewood dealer b Cambridge
Eliza Marshall wife 42 monthly nurse b Thetford Norfolk
Henry Marshall son 15 b Cambridge
Arthur T Marshall son 10 b Cambridge
Albert H Marshall son 6 b Cambridge
Olive Marshall 18 was a servant b Cambridge at 33 Chatsworth Rd Hackney in 1901
Ada 15 b Cambridge was a domestic nurse in Cherry Hinton Cambs
There's only one Gertrude Marshall birth registered in Cambs at the right time - Dec 1883 in Chesterton reg district vol 3b pafe 473. Equally there's only one Olive in Cambs and her birth was also registered in Chesterton in Sept 1882. Henry and Ada were also registered in Chesterton.
My money goes on the Gertrude age 17 that Ricky found working as a servant in Hackney, same as her sister Olive, and she gave her birthplace as Chesterton (whilst her age is clearly 17, Ancestry got the maths wrong as in the index they give her birth as 1882!). Whilst there were other Marshalls in Chesterton in 1891 I can't see a Gertrude
Your aunt or great aunt may have worked at Girton, but it just wasn't on 31 March 1901!
David