As to GRE or GMAT, most of the better (top 30) finance PhD's prefer GRE and some only take GRE while some take both. The lower ranked programs tend to either accept both or only accept GMAT. Take a look at individual school admissions websites. They usually have pretty good information there.
As to math, you will absolutely need calc 3 (multivariable calc) for any top 20 (and most top 50), and differential equations and real analysis are huge pluses and are pretty much necessary for a top 10 and highly suggested for a top 20. You can take courses in the fall and send your updated transcripts to the schools as soon as your grades are in. Admission committees meet in February or March at most schools, so if you can get in the updated transcripts by the end of January you should be fine. Otherwise you have the necessary lin alg and stats background, though if your intro stats course did not cover much probability stuff, a one semester course in calculus based probability could be useful, though by no means is it necessary for admission as long as you havethe intro stats course. If you get in calc 3 and one or both of differential equations or real analysis with good grades, you should be fine math wise for schools outside the top 10 and maybe even have a shot a top 10's (though a lot of students in top 10's have full undergrad math degrees and often math or engineering masters degrees so your math background is far weaker than theirs, but you would still have a shot).
Your weakness is the cumulative GPA from a LAC. You say you went to a top LAC, but unless you went to one of LACs that frequently send students to PhD programs, the adcoms will likely know little about whether your school grades tough or not. If your professors who are giving you
LORs are well known and well published in their fields and emphasize the tough grading at your school in the letter and discuss what it took to get economic honors at your school it will help a lot. Also, if your school frequently sends students to PhD programs, then the adcoms should have a good idea.
If the LAC you went to frequently sends students to top econ PhD's but not finance PhDs it might be good to also apply to some econ PhDs at schools with good finance people in the econ department, since the adcoms might have a better idea about your school's quality and toughness. (The math requirements for a top 20 econ and top 20 finance are similar.)
The teaching experience may help when it comes to getting funding if the funding is TAship based, but will probably not affect admissions much at most of the top schools.
Also, what was the other field you majored and did graduate work in? Depending on the field, your experience in that field may help admissions or may make no difference at all (it is highly unlikely that it will hurt).