"Te perdant je sens que..." translates to: "Losing you, I feel that..."
This phrase sets the tone for the poem, indicating that the speaker is experiencing deep sorrow and a sense of emptiness due to the loss of someone they love.
"Ce gagne à le connaître et ce souffre en perdant" translates to: "He gains by knowing him, and suffers by losing him."
This line further emphasizes the depth of the speaker's feelings. It highlights the value they placed on the person they lost, as the speaker acknowledges that others would also gain something valuable from knowing them. It also emphasizes the speaker's personal suffering caused by this loss.
The full context of the poem reveals the speaker's deep emotional distress at being separated from their beloved. This is further highlighted in the lines you provided, which convey both the value of the lost relationship and the intense suffering caused by the separation.