To list here the possible filters you can pass into VCFTools would be an exercise in redundancy. The documentation does a good job of laying them all out.
You're specifying the genotype quality filter correctly--are you saying that you're getting the same number of lines in your output file? If you're not sure, here's a quick way to check the total number of lines in the input and output files:
wc -l input.vcf
wc -l output.vcf
You should also check to see if the "GQ" tag is actually in your VCF file. Depending on what software was used to generated the VCF, there might not be Genotype Quality data for all sites, which is what VCFTools requires..