Any classical music fans?

[theoriginalspy]

If so, could you help me with my dilemma?

So I do the Secret Santa thing every year at school.  This year, under musical preferences, my person lists that she loves Mahler's work.  A bit of googling later, I learned his first name is Gustav.  So, if anyone knows who the heck he is and what works of his are popular, please, let me know.
Also, if you can think of a composer similar to Mahler that a Mahler fan would like, pass that along too please!