Practice : Ven. Mahasi Sayadaw
Is there any "Gatha" (Verse) can be used to contemplate on "Death"? The purpose is to use as an aid to generate necessary energy to continue practice [mostly as the first thing in the morning]
Because you mention the Mahasi Sayadaw in your question, most relevant is the Mahasi Sayadaw's discussion of the Purabheda Sutta:
(Sorry, the formatting is off in this version)
There are some Pali phrases in that book that might be of interest, e.g.:
Vītataṇho purābhedā, pubbamanta manissito.
Vemajjhe nupasaṅkheyo, tassa natthi purekkhataṃ.
The one who has removed craving before death, independent of past and future;
who is not stuck in the present; for such a one there is no further becoming.
Also in the Mahasi Sayadaw tradition, it is customary to reflect on death before starting the practice:
The fourth protection for your psychological benefit is to reflect on the phenomenon of ever-approaching death. Buddhist teachings stress that life is uncertain, but death is certain; life is precarious but death is sure. Life has death as its goal. There is birth, disease, suffering, old age, and eventually, death. These are all aspects of the process of existence.
The Pali for this is from the Dhammapada Commentary:
addhuvaṃ me jīvitaṃ, dhuvaṃ me maraṇaṃ
My life is unsure; my death is sure.
avassaṃ mayā maritabbameva,
Indeed, in the future I must die.
maraṇapariyosānaṃ me jīvitaṃ,
My life has death as its final end.
jīvitameva aniyataṃ, maraṇaṃ niyatanti
Life indeed is uncertain, death is certain.
-- Dhp-A 174 (pesakāradhītāvatthu)
These are often chanted before starting a meditation course in the Mahasi Sayadaw tradition.
Besides this, there are many verses about death in the tipitaka; the most common is that spoken by Sakka when the Buddha passed away:
“aniccā vata saṅkhārā, uppādavayadhammino.
uppajjitvā nirujjhanti, tesaṃ vūpasamo sukho”ti.
Impermanent indeed are all formations; of a nature to arise and pass.
Having arisen, they cease; their fading away is happiness.
-- DN 16
Another similar one we use often is Dhp 41:
aciraṃ vatayaṃ kāyo, pathaviṃ adhisessati.
chuddho apetaviññāṇo, niratthaṃva kaliṅgaraṃ.
In no long time indeed will this body lie on the earth;
Discarded, consciousness having departed, as useless as a charred log.
The Upajjhatthana Sutta five remembrances is used in the evening chanting by many Theravadin temples and monasteries.
Below are two English translations and the original Pali text of the "five remembrances":
The Buddha advised: "These are the five facts that one should reflect on often, whether one is a woman or a man, lay or ordained."
The Upajjhatthana Sutta translated by Thanissaro Bhikkhu
The actual gatha for chanting itself.
In The Dhammapada there are the Verse 46: The King of Death.
"Verse 46: One who knows that this body is impermanent like froth, and comprehends that it is insubstantial like a mirage, will cut the flowers of Mara (i.e., the three kinds of vatta or rounds), and pass out of sight of the King of Death."
Here is a video dhamma talk on The Dhammapada by Ven. Yuttadhammo. The video is about how one should practice in order for the King of Death not to be able to find one. The video is called Dhammapada Verse 46: Where the King of Death Cannot See.
There are also Verses 21, 22, 23: The Mindful Never Die.
"Verse 21: Mindfulness is the way to the Deathless (Nibbana); unmindfulness is the way to Death. Those who are mindful do not die; those who are not mindful are as if already dead."
"Verse 22: Fully comprehending this, the wise, who are mindful, rejoice in being mindful and find delight in the domain of the Noble Ones (Ariyas)."
"Verse 23: The wise, constantly cultivating Tranquillity and Insight Development Practice, being ever mindful and steadfastly striving, realize Nibbana: Nibbana, which is free from the bonds of yoga; Nibbana, the Incomparable!"
Here is another video dhamma talk on The Dhammapada by Ven. Yuttadhammo. The video is called Dhammapada Verses 21, 22, and 23: The Mindful Never Die.
In Aṅguttara Nikāya, Chakka Nipāta there are two discourses that helps the development of contemplation on death.
"... whoever develops mindfulness of death, thinking, 'O, that I might live for the interval that it takes to swallow having chewed up one morsel of food... for the interval that it takes to breathe out after breathing in, or to breathe in after breathing out, that I might attend to the Blessed One's instructions. I would have accomplished a great deal' — "
Maranassati Sutta: Mindfulness of Death (1)
"There is the case where a monk, as day departs and night returns, reflects: 'Many are the [possible] causes of my death. A snake might bite me, a scorpion might sting me, a centipede might bite me. That would be how my death would come about. That would be an obstruction for me. Stumbling, I might fall; my food, digested, might trouble me; my bile might be provoked, my phlegm... piercing wind forces [in the body] might be provoked. That would be how my death would come about. That would be an obstruction for me.'
"Further, there is the case where a monk, as night departs and day returns, reflects: [repitition] Maranassati Sutta: Mindfulness of Death (2)