How can I find the default string-to-key parameters in GPG? I am using it in Tails.
According to the Tails source, no custom S2K counts are specified for GnuPG. This means you will be using the default for the current version. The changelog for Tails shows that 3.7.1 is using both 2.1 and 1.4. For 2.1, the count seems to be variable and depends on your hardware's performance, with a minimum of 65536. Version 1.4 on the other hand hardcodes the default at 65536.
The S2K version will always be 3 as is the case for all modern GnuPG setups. As for the default digest algorithm, for both 2.1 and 1.4, SHA-1 is the default. Note that there is an open bug that may make it such that the various S2K options in 2.x do not do what you think they will do.