The phrase "biggest gainer" refers to someone or something that has experienced the largest positive change or improvement compared to others. Full definition
As is common for me with big gainers in my taxable account, I gave away the last bit to charity. (alephblog.com)
Owning additional stocks takes away the potential of big gainers significantly impacting your bottom line, as is the case with large mutual funds investing in hundreds of stocks. (investopedia.com)
Other big gainers included Funeral Services (8.3 %), Wireless Networking (7.7 %), Office Equipment & Supplies (6.5 %), Semiconductor (6.2 %), Railroad (6.0 %), and Entertainment Technology (6.0 %). (valueline.com)