This appears to be a known issue in API 18+:
https://code.google.com/p/android/issues/detail?id=59368 https://code.google.com/p/android/issues/detail?id=59700
The problem seems to occur when a TextView is part of a scrollable container (e.g. ListView), making the view ignore the vertical gravity for some reason (some sources suggest this has to do with the TextView being the child of a RelativeLayout, though it's been my experience that this can happen even when no such layout is involved).
A possible workaround (albeit not a particularly elegant one), would be to wrap the TextView in a LinearLayout. You can then use "layout_gravity" on the TextView to center it inside the LinearLayout, instead of relying on "gravity" (just make sure to wrap_content so the text itself is properly centered).
E.g., in your example:
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_alignBottom="@+id/seekbar"
android:layout_alignParentLeft="true"
android:layout_alignTop="@+id/seekbar"
android:layout_toLeftOf="@+id/seekbar"
android:layout_marginLeft="@dimen/margin_tiny_double" >
<com.package.custom.CustomTextFont
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:gravity="left"
android:paddingRight="@dimen/margin_tiny"
android:text="@string/text1"
android:textColor="@color/black"
android:textSize="@dimen/size_text_normal" />
</LinearLayout>
This method does have the disadvantage of adding an otherwise-unnecessary level to your view hierarchy, but it currently seems to be the only way around this (other than reverting to an earlier API level).
Also see similar question at: Android sdk 18 TextView gravity doesn't work vertically