Short answer. No. Article 97 of the Revised Penal Code, as amended by Republic Act No. 10592 (2013), states that an appeal by the accused shall not deprive him of entitlement to the good conduct time allowances described in the article. Filing an appeal does not forfeit the credits you would otherwise earn for good behavior.
What the law says
An appeal by the accused shall not deprive him of entitlement to the above allowances for good conduct.
Revised Penal Code, Article 97 — Allowance For Good Conduct. Read the full provision →
What the law says
During the first two years of imprisonment, he shall be allowed a deduction of twenty days for each month of good behavior during detention
Revised Penal Code, Article 97 — Allowance For Good Conduct. Read the full provision →
The appeal itself does not cost you the good conduct credit
Article 97 answers your question in its own closing sentence: an appeal by the accused shall not deprive him of entitlement to the above allowances for good conduct. The article ties the deductions to good behavior during detention or imprisonment, not to whether you chose to appeal. Exercising your right to appeal a conviction is not treated as a reason to strip you of the good conduct allowances the article otherwise grants.
The current scale, under Republic Act No. 10592
The deductions article 97 describes were revised by Republic Act No. 10592 in 2013, and it is that revised, current scale that governs. For the first two years of imprisonment alone, the article now provides: during the first two years of imprisonment, he shall be allowed a deduction of twenty days for each month of good behavior during detention. Larger deductions apply the longer the sentence runs, and an additional allowance is given for study, teaching, or mentoring service. Older reproductions of the Code still print a smaller pre-2013 scale, so it matters that this is the amount currently in force.
The credit is earned by conduct, not lost by procedure
Because the allowance is defined around good behavior during detention, an appeal, which is a procedural step in challenging the conviction, does not interrupt that entitlement. You can pursue your appeal and still accumulate the deductions the article describes for the time you actually spend in custody, since the two are governed by separate parts of the same provision rather than treated as trading off against each other.
Why this matters if you are deciding whether to appeal
It is a real, understandable worry that appealing might somehow cost you credit you would otherwise earn behind bars. Article 97 forecloses that specific concern directly, by name, in its final sentence. Whatever else factors into a decision to appeal, the loss of good conduct time allowances under this article is not one of the consequences the law attaches to it.