Three things:

1. Like others have already said, you need to enable access by unsafe apps in Gmail account 
settings. See [1].

2. In KMail POP3 settings you might need to prefix you POP3 username with "recent:" (without 
quotes but with colon), so for example in KMail settings my username is entered 
as "recent:jwstolarek@..." (again, without quotes). This enables checking email by multiple 
mail clients (in case you have several machines). See [2]. Moreover, for me without the "recent:" 
prefix, instead of having access to newest emails as they come in, I would only see email in a 
batch every few hours.

3. Contrary to what other said, I rarely have problems caused by logining in from different 
locations. I regularly use VPN so sometimes I'll be checking my email from Switzerland and 5 
minutes later I'm checking it from Canada. I have no problems in overwhelming majority of cases.

Janek

[1] https://support.google.com/accounts/answer/6010255?hl=en
[2] https://support.google.com/a/answer/6089246?hl=en