Don't resign. Let them terminate you. If you opt to voluntarily resign, you close the option of filing a case of illegal dismissal, later on.
Gross negligence has been defined as the want or absence of or failure to exercise slight care or diligence, or the entire absence of care. It evinces a thoughtless disregard of consequences without exerting any effort to avoid them.
In one case, the Supreme Court upheld the validity of dismissal on the ground of gross negligence even if the act complained of was not habitual. Thus, a bank employee was found grossly negligent when she delivered newly approved credit cards to a person she had not even seen before and she did not even ask for receipts, thereby enabling fictitious persons to use these cards, causing P740,000.00 loss to the bank. (See Citibank vs. Gatchalian, G.R. No. 111222, January 18, 1995.)
This case illustrates the utter lack of even the basic rudiments of diligence. That is why it is called "Gross Negligence". Habitual absenteeism and tardiness constitute gross and habitual neglect of duty. Repeated acts of absences without leave and frequent tardiness reflect indifferent attitude to and lack of motivation in work. These are examples of gross and habitual neglect.
I respectfully submit that, not being able to hit sales quota, in itself, is not gross and habitual neglect. The company must be able to show specific acts done habitually that indicate that there was an entire absence of care on your part. The Labor Code is a social legislation. That is why it contains this salutary reminder: "in case of doubt, resolve it in favor of labor".