It is correct. 'Put on 5 kilos' means the same as 'gained 5 kilos' and either can be used. 'Work holiday' is alright it just depends what you mean - a work holiday would mean that you were doing some work during the holiday e.g. you went to a conference at a resort (so it was both work and holiday). If you mean a holiday from work then as Antonio suggested you should remove 'work' from holiday.

it's fine, just remove 'work' from the holiday.

I think it should be : I gained five kilos since my holiday.