The material components of the gentle repose spell are:
a pinch of salt and one copper piece placed on each of the corpse's eyes, which must remain there for the duration
The rules for material components (PHB p. 203) state:
A character can use a component pouch or spellcasting focus in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.
The material component is not explicitly consumed by gentle repose, although the spell does specify with unusual detail how the components are to be used and how the components must remain with the body for the spell to function. This is different to the normal scenario where you just wave the component about then put it back into your pocket. The component is expected to actually do something.
While the material component is a thing with monetary value (because it is money), it does not explicitly specify a cost. Based on this question on detect thoughts (which also has a copper piece as a material component), I assume that the copper pieces do not count as having an indicated 'cost' for the purposes of being a material component.
If I have a spellcasting focus I can use, can I avoid the need to place copper pieces on each of the corpse's eyes? Or must I still put copper pieces on the eyes even if I have a spellcasting focus?
Due to repeated confusion, I thought I would say this outright. This question is not a duplicate of Do I need to hold an actual copper piece in order to cast Detect Thoughts? That question was concerned with the value of the copper pieces. However, this question is concerned with what you must do with the copper pieces. Detect thoughts does not specify what must be done with the copper pieces. Gentle repose specifies that something specific must be done with the copper pieces for the spell to work. Does the clause on what must be done with the material component, irrespective of its cost, void your ability to use a spellcasting focus?
Best Answer
There is no flavor text in D&D 5e Spells.
If the text says you have to do certain things, you have to do them. The text says to put the coins there, so you have to.
You can use a component pouch or spellcasting focus in place of the coins, as per the Material rules, if you prefer. But either way it has to be done in order for the spell to function.