If bitcoin price is $50000, you bought $1000 of bitcoin. As the price if bitcoin is increasing, the price of the coin you bought is also increasing. Taking below example for more clarity:
Bitcoin price Gain if $1000 worth of bitcoin is bought
$50000 $1000 No gain
$60000 $1200 $200 gain
$70000 $1400 $400 gain
$80000 $1600 $600 gain
$90000 $1800 $800 gain
$100000 $2000 $1000 gain
But if the price of bitcoin is reducing, that is how the loss will be also. The gain and loss is flexible with the price of bitcoin, slightest giqn or loss reflection even with the slightest changes.