If it’s a redirect to a hosted payment page with a gateway (customer enters card details onto Google Checkout’s secure pages for instance) then you need not worry.
All of the gateways you mention offer this, however I know PayPal (Pro) also has an API version where your server accepts the card details, then fires them off to PayPal to be processed, keeping your customer on your site all the way through for a more seamless process where customer journey and checkout flow are concerned.
If they are all hosted then so long as your business trades solely online, never coming into contact with card details (more crucially storing them, no matter for how short a time) then you are technically out of the scope of PCI. It can’t apply if you don’t touch card details.
Of course if you accept MOTO orders or process card details manually within the business, then unfortunately PCI is nowhere near as simple as the above… the more you process (the more card details you ‘touch’) the higher the level of certification you need… and it’s not cheap to jump through those hoops and get certified.
It’s all worth mentioning as many ecommerce software providers are finding their users absorbing huge costs to certify to a level which is often too high, often through misinterpretation. There’s an article here which simplifies PCI compliance a little where ecommerce businesses are concerned.