It depends. When you want to express, that Pizza is your favorit meal, you can say

Ich esse gern Pizza.

When you admit that you would accept your Pizza (probably already ordered or served) you would prefer

Ich esse Pizza gern.

In the first case, I would prefer "gerne" instead of "gern".

It depends. When you want to express that pizza is your favourite meal you can say:

Ich esse gern Pizza.

When you admit that you would be happy to eat pizza, e.g. that has already been ordered or served, you would say:

Ich esse Pizza gern.

In the first case, I would prefer "gerne" instead of "gern".

It depends. When you want to express, that Pizza is your favorit meal, you can say

Ich esse gern Pizza.

When you admit that you would accept your Pizza (probably already ordered or served) you would prefer

Ich esse Pizza gern.

I the first case I would prefer gerne instead of gern.

It depends. When you want to express, that Pizza is your favorit meal, you can say

Ich esse gern Pizza.

When you admit that you would accept your Pizza (probably already ordered or served) you would prefer

Ich esse Pizza gern.

In the first case, I would prefer "gerne" instead of "gern".